(a) Except as provided in subdivision (b), (c), or (d), a district that is authorized and elects to use the design-build process described in Chapter 4 (commencing with Section 22160) of Part 3 of Division 2 of the Public Contract Code for the construction of housing shall require that at least 20 percent of the residential units constructed be subject to a recorded affordability restriction for at least 55 years and be affordable to all of the following:

1. (1) Lower income households, as defined in Section 50079.5.
2. (2) Very low income households, as defined in Section 50105.
3. (3) Extremely low income households, as defined in Section 50106.
4. (4) Persons and families of low or moderate income, as defined in Section 50093.