FORM OF UNDERWRITING AGREEMENT

Exhibit 1.3

FORM OF UNDERWRITING AGREEMENT (ITERATIVE)

SG MORTGAGE SECURITIES, LLC

[              ] Asset-Backed [and Manufactured Housing Contract]

Notes, Series [              ]

UNDERWRITING AGREEMENT

[DATE]

[SG Americas Securities, LLC]

[                      ]

[                      ]

Ladies and Gentlemen:

SG Mortgage Securities, LLC, a Delaware limited liability company (the “Company”), proposes to sell to you (the “Underwriter”) the respective amounts set forth opposite your name in Schedule I attached hereto of [              ] Asset-Backed [and Manufactured Housing Contract] Notes, Series [              ], Class A-[1], Class A-[2], Class A-[3] and Class A-[4] (collectively, the “Notes”). The Notes together with the [            ] Home Loan Asset-Backed Certificates of the same series, will evidence the entire beneficial interest in the Trust Fund (as defined in the Indenture referred to below), consisting primarily of a pool (the “Pool”) of adjustable-rate first and junior lien one- to four-family residential home loans (the “Mortgage Loans”) as described in the Prospectus Supplement (as hereinafter defined) to be sold by the Company.

The Notes will be issued pursuant to an indenture (the “Indenture”) to be dated as of [DATE] (the “Cut-off Date”) among the Company, as depositor, [WELLS FARGO] (“Master Servicer”), as master servicer, and [TRUSTEE], as indenture trustee (the “Indenture Trustee”). The Notes are described more fully in the Base Prospectus and the Prospectus Supplement (each as hereinafter defined) which the Company has furnished to the Underwriter.

1. Representations, Warranties and Covenants .

1.1 The Company represents and warrants to, and agrees with the Underwriter that:

(a) The Company has filed with the Securities and Exchange Commission (the “Commission”) a registration statement (No. 333-[NUMBER]) on Form S-3 for the registration under the Securities Act of 1933, as amended (the “Act”), of [              ] Asset-Backed [and Manufacturing Housing Contract] Notes (issuable in series), including the Notes, which registration statement has become effective, and a copy of which, as amended to the date hereof, has heretofore been delivered to you. The Company proposes to file with the Commission pursuant to Rule 424(b) under the rules and regulations of the Commission under the Act (the “1933 Act Regulations”) a supplement dated [DATE] (the “Prospectus Supplement”), to the prospectus dated [DATE] (the “Base Prospectus”), relating to the Notes and the method of distribution thereof. Such registration statement (No. 333-[Number]) including exhibits thereto and any information incorporated therein by reference, as amended at the date hereof, is hereinafter called the “Registration Statement”; and the Base Prospectus and the Prospectus Supplement and any information incorporated therein by reference, together with any amendment thereof or supplement thereto authorized by the Company on or prior to the Closing Date (as defined herein) for use in connection with the offering of the Notes, are hereinafter called the “Prospectus.”

(b) The Registration Statement has become effective, and the Registration Statement as of the effective date (the “Effective Date,” as defined in this paragraph), and the Prospectus, as of the date of the Prospectus Supplement, complied in all material respects with the applicable requirements of the Act and the 1933 Act Regulations; and the Registration Statement, as of the Effective Date, did not contain any untrue statement of a material fact and did not omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading; and each Issuer Free Writing Prospectus (as defined herein) as of its date did not and at all times prior to the date of the Prospectus Supplement will not, and the Prospectus, as of the date of the Prospectus Supplement did not and as of the Closing Date will not, contain an untrue statement of a material fact and did not and will not omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading (except in the case of any Issuer Free Writing Prospectus, any omission with respect to information included in the definition of Senior Structure Information); provided , however , that neither the Company nor Master Servicer makes any representations or warranties as to the information contained in or omitted from the Registration Statement or the Prospectus or any amendment thereof or supplement thereto relating to the information therein that is Excluded Information (as defined herein); and provided , further , that neither the Company nor Master Servicer makes any representations or warranties as to either (i) any information contained in any Underwriter Prepared Issuer FWP (as defined herein) or Underwriter Free Writing Prospectus (as defined herein) except, in each case, to the extent of (x) any information set forth therein that constitutes Pool Information (as defined below) or (y) any information accurately extracted from any Issuer Free Writing Prospectus and included in any Underwriter Prepared Issuer FWP or Underwriter Free Writing Prospectus, or (ii) any information contained in or omitted from the portions of the Prospectus identified

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by underlining or other highlighting as shown in Exhibit E (the “Underwriter Information”). The Effective Date shall mean the earlier of the date on which the Prospectus Supplement is first used and the time of the first Contract of Sale (as defined herein) to which such Prospectus Supplement relates. The initial effective date of the Registration Statement was within three years of the Closing Date. If the third anniversary of the initial effective date occurs within six months after the Closing Date, the Company will use best efforts to take such action as may be necessary or appropriate to permit the public offering and sale of the Notes as contemplated hereunder. The Company acknowledges that the Underwriter Information constitutes the only information furnished in writing by you or on your behalf for use in connection with the preparation of the Registration Statement or the Prospectus, and the Underwriter confirms that the Underwriter Information is correct.

(c) (i) “ABS Informational and Computational Materials” shall have the meaning given such term in Item 1101 of Regulation AB.

(ii) “Approved Offering Materials” means with respect to any class of Notes anticipated to be rated in the highest category by any Rating Agency, collectively the following documents as most recently provided by the Company and designated in writing by the Company as Approved Offering Materials prior to the time of any Contract of Sale: (i) one or more term sheets, providing factual information about the Notes and the structure and basic parameters thereof (excluding information about the subdivision of the senior classes into tranches), the basic terms of the subordination or other credit enhancements if known, factual information about the Mortgage Loans (which may include parameters or “stips” or tabular data prepared by the Company), the identity of and basic information about key parties to the transaction known to the Company, and the tax, ERISA and SMMEA characteristics of the Notes, (ii) a term sheet supplement, containing risk factors and additional information of the type to appear in the Prospectus Supplement to the extent known, and (iii) the Base Prospectus, which may be provided by a weblink. Each of the items described in (i) and (ii) in the preceding sentence shall constitute an Issuer Free Writing Prospectus and any additional information provided by the Underwriter shall constitute an Underwriter Free Writing Prospectus or Underwriter Prepared Issuer FWP, as the case may be. With respect to any class of Notes anticipated to be rated in the second highest or a lower category by any Rating Agency, “Approved Offering Materials” means the Prospectus.

(iii) “Contract of Sale” has the same meaning as in Rule 159 of the 1933 Act Regulations and all Commission guidance relating to Rule 159.

(iv) “Excluded Information” shall mean, with respect to each of the Registration Statement and the Prospectus, the information identified by underlining or other highlighting as shown on Exhibit D.

(v) “Free Writing Prospectus” shall have the meaning given such term in Rules 405 and 433 of the 1933 Act Regulations.

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(vi) “Issuer Free Writing Prospectus” shall mean any Free Writing Prospectus prepared by or on behalf of the Company and identified by the Company as an Issuer Free Writing Prospectus and relating to the Notes or the offering thereof.

(vii) “Issuer Information” shall mean any information of the type specified in clauses (1) – (5) of footnote 271 of Commission Release No. 33-8591 (Securities Offering Reform), other than Underwriter Derived Information. Consistent with such definition, “Issuer Information” shall not be deemed to include any information in a Free Writing Prospectus solely by reason of the Company’s review of the materials pursuant to Section 4.4(e) below and, consistent with Securities Offering Reform Questions and Answers, November 30, 2005 promulgated by the staff of the Commission, “Issuer Information” shall not be deemed to include any information in a Free Writing Prospectus solely by reason that the Underwriter has agreed not to use such Free Writing Prospectus without consent of the Company.

(viii) “Permitted Additional Materials” shall mean information that is not ABS Informational and Computational Materials and (x) that are referred to in Section 4.4(c) so long as any Issuer Information provided by the Underwriter pursuant to Section 4.4(c) is limited to information included within the definition of ABS Informational and Computational Materials, (y) that constitute Note price, yield, weighted average life, subscription or allocation information, or a trade confirmation, or (z) otherwise with respect to which the Company has provided written consent to the Underwriter to include in a Free Writing Prospectus.

(ix) “Pool Information” means with respect to any Free Writing Prospectus, the information with respect to the characteristics of the Mortgage Loans and administrative and servicing fees, as provided by or on behalf of the Company or Master Servicer to the Underwriter at the time most recent to the date of such Free Writing Prospectus.

(x) “Senior Structure Information” shall mean, with respect to each class of Notes anticipated to be rated in the highest category by any Rating Agency (collectively, the “Senior Notes”), (i) the Note Rate if a fixed rate, or the formula for determining the Note Rate, (ii) the terms and the provider of any yield maintenance agreement, swap agreement or other agreement that provides payments payable on any class of the Senior Notes, (iii) the terms and the provider of any surety bond, financial guaranty insurance policy, or other insurance policy regarding any class of the Senior Notes not known to the Company when the Approved Offering Materials were prepared, (iv) the allocation to each class of Senior Notes of the aggregate amount of the cashflow payable among the Senior Notes collectively, and (v) the allocation to each class of Senior Notes of the aggregate amount of any Realized Losses allocable to the Senior Notes collectively.

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(xi) “Underwriter Derived Information” shall refer to information of the type described in clause (5) of footnote 271 of Commission Release No. 33-8591 (Securities Offering Reform) when prepared by the Underwriter, including traditional computational and analytical materials prepared by the Underwriter.

(xii) “Underwriter Free Writing Prospectus” shall mean all Free Writing Prospectuses prepared by or on behalf of the Underwriter other than any Underwriter Prepared Issuer FWP, including any Permitted Additional Materials.

(xiii) “Underwriter Prepared Issuer FWP” shall mean any Free Writing Prospectus or portion thereof prepared by or on behalf of the Underwriter that contains only a description of the final terms of the Notes or of the offering of the Notes after the final terms have been established for all classes of Senior Notes.

(xiv) “Written Communication” shall have the meaning given such term in Rule 405 of the 1933 Act Regulations.

(d) The Company has been duly formed and is validly existing as a limited liability company in good standing under the laws of the State of Delaware and has the requisite power to own its properties and to conduct its business as presently conducted by it.

(e) The Company was not, as of any date on or after which a bona fide offer (as used in Rule 164(h)(2) of the 1933 Act Regulations) of the Note is made an Ineligible Issuer, as such term is defined in Rule 405 of the 1933 Act Regulations. The Company shall comply with all applicable laws and regulations in connection with the use of Free Writing Prospectuses, including but not limited to Rules 164 and 433 of the 1933 Act Regulations and all Commission guidance relating to Free Writing Prospectuses, including but not limited to Commission Release No. 33-8591.

(f) This Agreement has been duly authorized, executed and delivered by the Company.

(g) As of the Closing Date (as defined herein), the Notes will conform in all material respects to the description thereof contained in the Prospectus and the representations and warranties of the Company in the Indenture will be true and correct in all material respects.

1.2 Master Servicer represents and warrants to, and agrees with the Underwriter that as of the Closing Date the representations and warranties of Master Servicer in the Servicing Agreement will be true and correct in all material respects.

1.3 The Underwriter represents and warrants to and agrees with the Company and Master Servicer that:

(a) The Underwriter has no present knowledge or expectation that it will be unable to pay any United States taxes owed by it so long as any of the Notes remain outstanding.

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(b) The Underwriter has no present knowledge or expectation that it will become insolvent or subject to a bankruptcy proceeding for so long as any of the Notes remain outstanding.

(c) The Underwriter hereby certifies that (i) with respect to any classes of Notes issued in authorized denominations or Percentage Interests of less than $[              ] or [      ]%, as the case may be, the fair market value of each such Note sold to any person on the date of initial sale thereof by the Underwriter will not be less than $[              ] and (ii) with respect to each class of Notes to be maintained on the book-entry records of The Depository Trust Company (“DTC”), the interest in each such class of Notes sold to any person on the date of initial sale thereof by the Underwriter will not be less than the minimum denomination indicated for such class of Notes in the Prospectus Supplement.

(d) The Underwriter will have funds available at the Trustee, in the Underwriter’s account at such bank at the time all documents are executed and the closing of the sale of the Notes is completed, except for the transfer of funds and the delivery of the Notes. Such funds will be available for immediate transfer into the account of Master Servicer maintained at such bank.

(e) As of the date hereof and as of the Closing Date, the Underwriter has complied with all of its obligations hereunder and all information contained in any Underwriter Free Writing Prospectus and in any Underwriter Prepared Issuer FWP as used in connection with any Contract of Sale and all Underwriter Information are accurate in all material respects (taking into account the assumptions explicitly set forth in such Underwriter Prepared Issuer FWP or Underwriter Free Writing Prospectus), except to the extent of (x) any errors therein that are caused by errors or omissions in the Pool Information or (y) information accurately extracted from any Issuer Free Writing Prospectus and included in any Underwriter Prepared Issuer FWP or Underwriter Free Writing Prospectus.

(f) Prior to the Closing Date, the Underwriter shall notify the Company and Master Servicer of the earlier of (x) the date on which the Prospectus Supplement is first used and (y) the time of the first Contract of Sale to which such Prospectus Supplement relates.

2. Purchase and Sale . Subject to the terms and conditions and in reliance upon the representations and warranties herein set forth, the Company agrees to sell to the Underwriter, and the Underwriter agrees to purchase from the Company, the Notes set forth opposite its name in Schedule I hereto, at a price equal to 100% of the aggregate principal balance of the Class A-[1], Class A-[2], Class A-[3] and Class A-[4] Notes, as of the Closing Date.

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3. Delivery and Payment . Delivery of and payment for the Notes shall be made at the office of Mayer, Brown, Rowe & Maw LLP at 10:00 a.m., New York City time, on [DATE] or such later date as you shall designate, which date and time may be postponed by agreement between you and the Company (such date and time of delivery and payment for the Notes being herein called the “Closing Date”). Delivery of the Notes shall be made to you through The Depository Trust Company (“DTC”).

4. Offering by Underwriter .

4.1 It is understood that the Underwriter proposes to offer the Notes for sale to the public as set forth in the Prospectus and the Underwriter agrees that all such offers and sales shall be made in compliance with all applicable laws and regulations. Prior to the date of the first Contract of Sale made based on the Approved Offering Materials, the Underwriter has not pledged, sold, disposed of or otherwise transferred any Note, Mortgage Loans or any interest in any Note.

4.2 It is understood that you will solicit offers to purchase the Notes as follows:

(a) Prior to the time you have received the Approved Offering Materials you may, in compliance with the provisions of this Agreement, solicit offers to purchase Notes; provided, that you shall not accept any such offer to purchase a Note or any interest in any Note or Mortgage Loan or otherwise enter into any Contract of Sale for any Note, any interest in any Note or any Mortgage Loan prior to your conveyance of Approved Offering Materials to the investor.

(b) any Written Communication relating to the Notes made by an Underwriter in compliance with the terms of this Agreement prior to the time such Underwriter has entered into a Contract of Sale for Notes with the recipient shall prominently set forth the following statements (or a substantially similar statements approved by the Company):

The information in this free writing prospectus, if conveyed prior to the time of your contractual commitment to purchase any of the Notes, supersedes any information contained in any prior similar materials relating to the Notes. The information in this free writing prospectus is preliminary, and is subject to completion or change. This free writing prospectus is being delivered to you solely to provide you with information about the offering of the Notes referred to in this free writing prospectus and to solicit an offer to purchase the Notes, when, as and if issued. Any such offer to purchase made by you will not be accepted and will not constitute a contractual commitment by you to purchase any of the Notes, until we have accepted your offer to purchase Notes.

The Notes referred to in these materials are being sold when, as and if issued. The issuer is not obligated to issue such Notes or

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any similar security and the underwriter’s obligation to deliver such Notes is subject to the terms and conditions of the underwriting agreement with the issuer and the availability of such Notes when, as and if issued by the issuer. You are advised that the terms of the Notes, and the characteristics of the mortgage loan pool backing them, may change (due, among other things, to the possibility that mortgage loans that comprise the pool may become delinquent or defaulted or may be removed or replaced and that similar or different mortgage loans may be added to the pool, and that one or more classes of Notes may be split, combined or eliminated), at any time prior to issuance or availability of a final prospectus. You are advised that Notes may not be issued that have the characteristics described in these materials. The underwriter’s obligation to sell such Notes to you is conditioned on the mortgage loans and Notes having the characteristics described in these materials. If for any reason the issuer does not deliver such Notes, the underwriter will notify you, and neither the issuer nor any underwriter will have any obligation to you to deliver all or any portion of the Notes which you have committed to purchase, and none of the issuer nor any underwriter will be liable for any costs or damages whatsoever arising from or related to such non-delivery.

4.3 It is understood that you will not enter into a Contract of Sale with any investor until the Approved Offering Materials have been conveyed to the investor with respect to the Notes which are the subject of such Contract of Sale.

4.4 It is understood that you may prepare and provide to prospective investors certain Free Writing Prospectuses, subject to the following conditions:

(a) Unless preceded or accompanied by a prospectus satisfying the requirements of Section 10(a) of the Act, the Underwriter shall not convey or deliver any Written Communication to any person in connection with the initial offering of the Notes, unless such Written Communication (i) is made in reliance on Rule 134 under the Act, (ii) constitutes a prospectus satisfying the requirements of Rule 430B under the Act or (iii) constitutes a Free Writing Prospectus (as defined in Section 1.1(c) above) consisting solely of (x) information of a type included within the definition of ABS Informational and Computational Materials (as defined below), (y) Permitted Additional Materials or (z) information accurately extracted from any Issuer Free Writing Prospectus and included in any Underwriter Prepared Issuer FWP or Underwriter Free Writing Prospectus.

(b) The Underwriter shall comply with all applicable laws and regulations in connection with the use of Free Writing Prospectuses, including but not limited to Rules 164 and 433 of the 1933 Act Regulations and all Commission guidance relating to Free Writing Prospectuses, including but not limited to Commission Release No. 33-8591.

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(c) It is understood and agreed that all information provided by the Underwriter to or through Bloomberg or Intex or similar entities for use by prospective investors, or imbedded in any CDI file provided to prospective investors, to the extent constituting a Free Writing Prospectus, shall be deemed an Underwriter Free Writing Prospectus.

(d) All Free Writing Prospectuses provided to prospective investors, whether or not filed with the Commission, shall bear a legend including the following statement (or a substantially similar statement approved by the Company):

“THE DEPOSITOR HAS FILED A REGISTRATION STATEMENT (INCLUDING A PROSPECTUS) WITH THE SECURITIES AND EXCHANGE COMMISSION (THE SEC) FOR THE OFFERING TO WHICH THIS COMMUNICATION RELATES. BEFORE YOU INVEST, YOU SHOULD READ THE PROSPECTUS IN THAT REGISTRATION STATEMENT AND OTHER DOCUMENTS THE DEPOSITOR HAS FILED WITH THE SEC FOR MORE COMPLETE INFORMATION ABOUT THE DEPOSITOR AND THE OFFERING. YOU MAY GET THESE DOCUMENTS AT NO CHARGE BY VISITING EDGAR ON THE SEC WEB SITE AT WWW.SEC.GOV. ALTERNATIVELY, THE DEPOSITOR, ANY UNDERWRITER OR ANY DEALER PARTICIPATING IN THE OFFERING WILL ARRANGE TO SEND YOU THE PROSPECTUS AT NO CHARGE IF YOU REQUEST IT BY CALLING TOLL-FREE 1–8[XX–XXX–XXXX] OR VIA EMAIL AT                      .

Each of the Underwriter and the Company shall have the right to request additional specific legends or notations to appear on any Free Writing Prospectus and shall have the right to require changes regarding the use of terminology and the right to determine the types of information appearing therein with the approval of the other (which shall not be unreasonably withheld).

(e) The Underwriter shall deliver to the Company and its counsel (in such format as reasonably required by the Company), no later than the business day prior to the date of the required filing under Section 5.10, an Underwriter Prepared Issuer FWP. To facilitate filing to the extent required by Section 5.10 or 5.11, as applicable, all Underwriter Derived Information shall be set forth in a document separate from any Underwriter Prepared Issuer FWP including Issuer Information.

(f) The Underwriter shall provide the Company with a letter from [ACCOUNTING FIRM], certified public accountants, prior to the Closing Date, satisfactory in form and substance to the Company, Master Servicer and their respective counsels and the Underwriter, to the effect that such accountants have performed certain

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specified procedures, all of which have been agreed to by the Company and the Underwriter, as a result of which they determined that certain information of an accounting, financial or statistical nature that is included in any Underwriter Prepared Issuer FWP, other than any Pool Information therein and any information accurately ext