Most building codes adopt ICC/ANSI A117.1, which tracks very closely with the ADA insofar as the physical/technical requirements. The ADA requirements can be found on-line, whereas to get the A117.1 requirements you have to buy the book.
ADA Showers:
https://www.ada.gov/law-and-regs/design-standards/2010-stds/#608-shower-compartmentsThere are basically three types: one 36" x 36" transfer shower stall, and two roll-in versions. The general rule as applied in my state is that the interior dimensions of accessible shower stalls are absolute -- to do anything with alternate dimensions requires a specific variance from the state, and I know of a case where a building inspector rejected a shower because it was built to 36" x 36" clear between the cement board backer sheets, but the tile reduced the inside dimension to 35".
That said, of the two roll-in showers one is 60" x 30" and the other is 60" x 36" with a reduced-width entry. I think with an available space of 55" x 55" it should be possible to configure a shower with a fold-up seat that allows sufficient clear space to maneuver a wheelchair into and out of the shower stall. But the seat is not required by the ADA in roll-in showers in private residences, only in hotels.
608.4 Seats. A folding or non-folding seat shall be provided in transfer type shower compartments. A folding seat shall be provided in roll-in type showers required in transient lodging guest rooms with mobility features complying with 806.2. Seats shall comply with 610.
EXCEPTION: In residential dwelling units, seats shall not be required in transfer type shower compartments provided that reinforcement has been installed in walls so as to permit the installation of seats complying with 608.4.
The 2009 edition of the A117.1 standard is mostly the same, except for the language of the exception. The strict application of the dimensions is more important in a transfer type shower stall, because you don't want to have the user transfer onto a seat in the stall and then not be able to reach the controls. All the requirements are predicated on the assumption of a single person using the facility unassisted. For a private home, the requirements don't call for the seat but they do require that blocking be installed so a seat can be added later. With all the blocking that's needed for the various grab bars, a bit more at the seat location shouldn't be a deal breaker.
Who are you talking to at the VA? At the local VA hospital where I go they have people who I think are titled as social workers, and such people might be able to help you understand if there's any flexibility in the application of the ADA.
