For Section 8, Public Housing, and other affordable housing subsidy programs, your monthly rent payment is generally 30% of the household's monthly gross income, but there are other factors that can determine your rent.
Affordable properties that participate in programs without rental subsidies, such as Low-Income Housing Tax Credit properties, offer a flat monthly rent that is below the market rate. For example, an affordable housing property may charge all tenants $500 for a 1-bedroom unit in an area where the Fair Market Rent (FMR) for a 1-bedroom unit is $700.
In regards to the Section 8 Housing Choice Voucher program, it is important to find a unit that is below or near the FMR rate of that area. For example, if you find a 3-bedroom unit going for $1,400 a month, but the FMR of a 3-bedroom unit is $1,100, you may be required by the housing authority to cover that extra $300. If the monthly rent is over the FMR by just a small amount (tens of dollars) the housing authority may allow you to still rent it without paying extra money. However, there is no guarantee that you would be granted by this accommodation.