The HUD-code has the following requirements for an attached garage for a manufactured home at 24CFR 3280.212:

(a) When a manufactured home is designed for factory construction with an attached garage or is designed for construction of an attached site-built garage that is not self-supported, the manufacturer must design the manufactured home to accommodate all appropriate live and dead loads from the attached garage structure that will be transferred through the manufactured home structure to the home's support and anchoring systems. (Also see What is a host beam at a mobile/manufactured home?)

(b) The design must specify the following home and garage characteristics including maximum width, maximum sidewall height, maximum roof slope, live and dead loads, and other design limitations or restrictions using loads provided by this Code. 

(c) When a manufactured home is factory constructed with an attached garage or is constructed for the attachment of a site-built garage, provisions must be made to provide fire separation between the garage and the manufactured home. 

(1) The garage must be separated from the manufactured home and its attic by not less than 1⁄2-inch gypsum board or equivalent applied to the garage side of the manufactured home, separation shall be from the underside of the floor to the underside of the roof deck and may be provided on-site as part of an On Site Completion of Construction approval. Garages beneath habitable rooms must be separated from all habitable rooms by 5⁄8-inch, Type X gypsum board or equivalent. Where the separation is a floor ceiling assembly, the structure supporting the separation must also be protected by not less than 1⁄2-inch gypsum board or equivalent. The design approval and the manufacturer's installation instructions must also include provision for equivalent vertical or horizontal separation between the garage and the manufactured home as appropriate. 

(2) [Reserved] 

(d) Openings from a garage directly into a room designated for sleeping purposes are not permitted. 

(e) Other openings between the garage and the manufactured home must: 

(1) Be equipped with solid wood doors not less than 13⁄8 inch in thickness, or solid or honeycomb steel doors not less than 13⁄8 inch in thickness, or 20-minute fire-rated doors, and all doors shall be of the self-closing type; and 

(2) Be in addition to the two exterior doors required by 3280.105.

(f) Ducts penetrating the walls or ceilings separating the manufactured home from the garage must be constructed of a minimum No. 26 gauge steel or other approved material and must have no openings into the garage. 

(g) Installation instructions shall be provided by the home manufacturer that, in addition to addressing the fire separation as required in this section, shall identify acceptable attachment locations, indicate design limitations for the attachment of the garage including acceptable live and dead loads for which the home has been designed to accommodate, and provide support and anchorage designs as necessary to transfer all imposed loads to the ground in accordance with  3285.301 and 3285.401 of this chapter. 

(h) A site-built, self-supported garage is considered an add-on, per 3282.8(j)(1), that does not affect the ability of the manufactured home to comply with the Construction and Safety Standards. The design and construction of the garage is subject to state and or local authorities having jurisdiction.