(CNN) Are San Francisco families earning $117,400 a year really considered low income?

That's what the latest data from the US Housing and Urban Development Department seems to suggest.

for a family of four, requires a bit more explanation. No doubt San Francisco has seen both its incomes and home prices soar in recent years, thanks largely to money flooding in from the tech boom. But the figure from HUD, which is incomefor a family of four, requires a bit more explanation.

Every year, HUD sets income limits that determine who can qualify for housing assistance, including Section 8 vouchers, public housing and other assistance programs. The calculation is also used to set eligibility for affordable housing built by developers who receive tax credits.

The formula takes into account an area's median family income, as well as its housing costs. Those who make 80% of the formula amount are considered "low income," while those earning 50% are "very low income" and those making 30% are considered "extremely low income."

Read More