Can I Build an ADU (Casita or Garage Apartment) in San Antonio?

Here is the money problem hiding behind the question: a casita can put rent in your pocket every month, but if it is built without a permit, on the wrong side of a setback, or financed the wrong way, that same casita can cost you real money in fines, rework, and a financing denial at the worst possible moment. That is why the rulebook comes before the contractor, Patriot Nation. This page walks through the city's actual rules and the financing reality behind them, so let's kick off this rodeo.

An accessory dwelling unit, or ADU, is a second, smaller home on the same lot as a main house. In San Antonio people call them casitas and garage apartments. They can be attached or detached, and the appeal is usually the same: an income-producing rental, a place for family, or both, on land you already own.

I am a loan officer and a Realtor, so I get asked about casitas from both sides. Buyers want to know if a house with a casita can help them qualify, and owners want to know if their lot can take one and how to pay for it. Here is the current rulebook, straight from the City of San Antonio, and the financing reality behind it.

Are ADUs Legal in San Antonio?

Yes, citywide and by right in most residential districts. San Antonio's Unified Development Code, Section 35-371 (Accessory Dwellings), permits one accessory dwelling unit on a lot with a primary residence in the R-1 through R-6 single-family and residential districts, the RM-4 through RM-6 districts, and the multi-family districts.^[1] The amendments that made ADUs broadly legal took effect January 1, 2023.^[2] City of San Antonio Development Services

• One ADU per lot. A lot with a primary residence can hold one accessory dwelling unit.
• Rear yard placement. The ADU is located in the rear yard on the lot.
• Watch for deed restrictions. City zoning allows the ADU, but a neighborhood's deed restrictions or an HOA's rules can still prohibit or limit one. Check your deed restrictions and HOA covenants before you commit.

The short version, Patriot Nation: the city says yes, but your deed restrictions and HOA get a vote too. Read those documents before you fall in love with a plan, because a casita you cannot build is just an expensive daydream.

The Size and Height Limits

The official numbers come from the City of San Antonio Development Services ADU fact sheet (February 2023):^[2] City of San Antonio Development Services

• Gross floor area: the ADU is limited to 800 square feet, or 50% of the primary structure's gross floor area if that is larger, capped at 1,600 square feet. The effect is that every lot gets at least 800 square feet to work with.
• Height: generally capped at 25 feet or two stories.

One honest warning: third-party guides quote different size and setback figures for San Antonio, and some are simply wrong or outdated. The city's own fact sheet and the ordinance text are the numbers to trust, and the permit review at Development Services is the final word for your property.

And here is the money angle: every square foot you add is square footage you pay for in concrete, framing, and finishes. The smart build is the smallest casita that does the job, not the biggest one the code allows, and that discipline is what keeps the project from eating your equity before it ever rents.

The Owner-Occupancy Rule

San Antonio requires the property owner to live on the lot: either in the principal home or in the ADU.^[1] City of San Antonio That owner-occupied unit may not be rented, and the arrangement is enforced with a notarized affidavit and a recorded covenant that runs with the property.

This matters for financing and for resale. If the owner does not live on site, the property does not meet the owner-occupancy rule and must be rezoned, which is a longer and less predictable path. If you buy a home planning to live in the main house and rent the casita, that setup fits the rule. If your plan was to rent both units, check the rezoning route before you make an offer.

Plain English: you live on the lot, or the casita does not get built. That one sentence decides whether your plan is a legal income play or a rezoning project, so know it before you write an offer.

Setbacks and Your Address: Check by Address

The general setback standard for a detached ADU is 3 feet from the side and rear property lines, with no eave overhang and fire-rated walls when the structure sits within 3 feet of a property line.^[3] City of San Antonio Development Services, Information Bulletin IB402

But that is the general rule, not your answer. The ADU also has to meet the primary structure's setback requirements for the underlying zoning district, and corner lots, utility easements, and lot shape all change the math. That is why the only reliable step is a check by address: confirm the zoning, the lot's setback situation, and the flood and design requirements with the City of San Antonio Development Services or your permitted engineer before you spend on plans.

A little due diligence here beats a permit denial after you have already paid for plans. In the real world, permit problems are what turn a casita project from a money-maker into a money pit, so spend the time on the front end.

What Does an ADU Cost to Build?

There is no single number, because size, foundation type, utilities, and finish level move the total a lot. The published estimates below are third-party planning figures, not quotes: detached ADUs in San Antonio typically run about $80,000 to $150,000 to build, with design and drawings adding roughly $3,000 to $8,000 and separate utility connection fees on top.^[4] Steadily, Texas ADU laws by city, 2026 Permit fees through the city run in the low thousands and depend on the project's size and scope.

A real number for your lot requires three things: a site-specific plan, a permit fee estimate from Development Services, and bids from at least two licensed contractors. Anyone who quotes you a firm price without those steps is guessing.

How Do People Finance an ADU?

Most ADUs in San Antonio are paid for with equity or renovation financing, not a brand-new mortgage product. The realistic paths:

• Cash-out refinance, HELOC, or home equity loan. The most common route for owners with existing equity. Your rate and amount depend on the loan to value, credit, and income picture.
• Renovation and construction loans. Programs such as FHA 203(k) and Fannie Mae HomeStyle let you finance the build into one loan, and a construction-to-permanent loan works for a new detached casita.
• Casita Incentive Pilot Program. The City of San Antonio approved this program on August 22, 2025. It offers no-interest, forgivable loans to build or rehab a casita: up to $25,000 over 5 years, or $35,000 over 10 years, for owners who rent the casita to a household at or below 80% of Area Median Income, with fee waivers on city development and permitting and on SAWS water and wastewater impact fees.^[5] City of San Antonio, Casita Incentives Pilot Program

Because I hold both a mortgage license and a real estate license, I can show you the financing side and the zoning side in one conversation. Start with the property's equity position and the casita program's income limits, then pick the loan structure that fits.

Can ADU Rental Income Count Toward a Mortgage?

Yes. Fannie Mae, Freddie Mac, and FHA all allow lenders to count ADU rental income toward qualification.^[6] Dwelling Index, 2026 The conditions matter more than the headline:

• The unit must be legal and permitted, with a certificate of occupancy. An unpermitted casita does not qualify, full stop.
• History or a lease. For an existing casita, lenders typically want 12 to 24 months of documented rent history from tax returns or bank deposits. For a newer unit, an executed lease with a real tenant can be enough.
• Lenders commonly count 75% of the rent toward income, to cover vacancy and maintenance, and Fannie Mae caps ADU income at 30% of your total qualifying income.^[6] Dwelling Index, 2026

If you are shopping for a home with an existing casita, that rental picture can be part of your pre-approval from day one, instead of a surprise at the end. I model the property's actual rent history, the market rent, and the loan limits together so the number you pre-qualify for matches the house you can really afford.

That is what being on your side looks like: the zoning answer and the financing answer in one conversation, before you sign anything. Now make it a great one.

Quick FAQ on ADUs in San Antonio

Sincerely,
Patrick Kevin Fagan

This page is education, not legal, zoning, or tax advice. Zoning, setback, and permit rules change and are address-specific, so confirm every item with the City of San Antonio Development Services before building. Program details for the Casita Incentive Pilot Program are set by the City of San Antonio and change by funding cycle.