ADA applies if any of the following conditions apply:
(1) Public housing (as defined) for example housing that is owned, administered or operated on behalf of a public entity or a public entity's programs and services (shelter / supportive housing, low income housing government programs, etc.);
(2) Project funding from public subsidy (low income housing tax credits, HUD, Block Grant, Municipal Bond funding, etc.);
(3) Pool area is made open to the public (not limited to tenant's, their guests and invitees) - making it a public accommodation.
In California, if it is public housing as defined, then CBC Chapter 11B accessibility requirements apply to the common use pool area and the path of travel elements (pedestrian routes, parking, bathing facilities, signage, etc. serving the pool area are covered by accessibility standards (doors, gates and gate hardware 11B-404 with an exception for gate hardware as )
FHA applies if built as covered multifamily housing for first occupancy on or after 13 March 1991 (4 or more dwelling units) - plus common use areas are covered and must be accessible (ingress and egress)
CBC applies in California if built as covered multifamily housing for first occupancy on or after 13 March 1991 (3 or more dwelling units) - plus common use areas are covered and must be accessible (ingress and egress). FHA and Chapter 11A both apply to privately owned, privately funded covered multifamily housing. FHA, ADA and CBC 11B apply to public housing.
In California, a change in occupancy is an alteration, as defined, but does not necessarily trigger multifamily accessibility requirements - evaluated on case-by-case basis.
Additions to common use areas of privately owned, privately funded covered multifamily housing built prior to March 13, 1991. The addition, say for example a swimming pool are subject to FHA and CBC building code.
If a swimming pool was included in the original construction of public housing on or after 13 March 1991, then the 1992 ADAAG would apply and the 1989 California Building Code, as amended, would apply. As of March 2012, the 2010 ADA (2004 ADAAG) applies, and entirely new sections of the code with specifica application to pools and spas became enforceable.
See CBC Chapter 31B establishes requirements for pool enclosures, gates and gate hardware.
Egress is subject to Chapter 10 and occupant load factor calculations for direction of swing of doors and gates.
The puzzle for evaluating which code sections apply takes a bit of careful permit history research, code and case law research, and an appetite for loosening, if not untangling, the code knot for such facilities. When in doubt, protect life safety first, it is usually not expensive enough to justify not implementing. As far as accessibility requirements, the puzzle is daunting to solve, but can be methodically resolved.
Cheers