Cities | San Antonio

San Antonio ADU and Small Lot Rules 2026

San Antonio writes its land rules in one book, the Unified Development Code (UDC). It gives a lot two, three or four homes through its RM mixed residential districts, lets lots in its R and RM districts add one accessory dwelling unit (ADU) if the owner lives on site, and since September 4, 2025 carries the state's SB 15 small-lot rules in its own text. This page gives the numbers for each, with the code section that sets them.

State Law in San Antonio, in Brief

San Antonio had 1,434,625 people in the 2020 Census, and Bexar County had 2,009,324, so the city is among the 19 that the 2025 laws reach, by VanPlex count from 2020 Census figures. SB 15 limits the lot size the city can require on new single-family tracts of five acres or more with no recorded plat, and San Antonio copied those limits into its UDC. SB 840 requires the city to allow housing of three or more units on land zoned for office, commercial, retail, warehouse or mixed use, with no rezoning, except within 3,000 feet of an airport or military base. HB 24 changed the protest rules for rezonings that add housing, which matters if you ask to move a lot from R-4 to RM-4. Texas has no statewide ADU law, so the ADU rules below are San Antonio's own.

Lot Sizes and Homes by District

Table 310-1 in the UDC sets the base lot size and height for each residential district; R-4 and RM-4, for example, both start at 4,000 square feet. The table below adds the number of units the RM districts allow, from UDC 35-310.06, and whether the City lists the district for ADUs.

Sources: San Antonio UDC Article III, Division 2, Table 310-1 (history ends Ord. No. 2025-09-04-0580), read through UDC 35-310.05a; UDC 35-310.06; UDC 35-371.

The ADU column follows the City's own list of districts that allow ADUs: "R-1, R-2, R-3, R-4, R-5, R-6, RM-4, RM-5, RM-6 and in multi-family districts up to a total of five units," from the City's ADU page. Table 310-1 also lists maximum lot sizes for R-2 and R-1, printed as "2.999" and "1.999", which appear to be typing errors, so this page leaves them out. Read the current table on Municode before you plat lots near those limits.

Two to Four Homes: The RM Districts

RM stands for mixed residential. UDC 35-310.06(a)(1) says: "The maximum number of dwellings is limited to two (2) units for RM-6, three (3) units for RM-5, and four (4) units for RM-4." The units may go beyond the table density when the lot meets the district's minimum lot size. These rules come from Ordinance No. 2022-11-03-0831, in force since January 1, 2023.

Two design rules apply on smaller lots. On a lot of one-third of an acre or less, "multi-unit construction ... shall be within a single structure," so a fourplex on a 4,000 square foot RM-4 lot is one building with four units. Under Note 13 of Table 310-1, a building with two to four units on such a lot needs a front entry facing the street and a front walkway.

A building with three or four stacked units, or units that share exits, falls under the commercial building code in Texas. The state's residential code covers one- and two-family homes and townhouses of up to three stories with separate exits. The building codes page explains the line.

Small-Lot Districts: R-3, R-2 and R-1

UDC 35-310.05a describes R-3, R-2 and R-1 as districts for small-lot single-family homes, townhouses and zero-lot-line homes. A zero-lot-line home sits on, or right next to, one side property line, with no side yard on that side. An R-3 subdivision is capped at 30 acres.

The same section lets builders mix lot sizes inside an RM development. In its words, "a limited number of lots may be developed as "R-3", "R-2", and "R-1" lots by right without additional zoning provided the total number of "R-3" "R-2", and "R-1" sized lots do not exceed fifteen (15) percent of the gross number of "RM-4," "RM-5" and "RM-6" lots within the development." By right means approved by staff against the written rules, with no rezoning vote.

Accessory Dwelling Units

An ADU is a second, smaller home on the same lot as a house, such as a backyard cottage or garage apartment. San Antonio's rules are in UDC 35-371, amended by Ordinance No. 2022-11-03-0831, effective January 1, 2023.

Sources: UDC 35-371; fire rating from the City of San Antonio ADU page.

The owner rule decides who can use this route. The code says the owner "must occupy either the principal unit or the accessory dwelling as their permanent residence, and shall at no time receive rent for the owner-occupied unit." So on a lot with a house and an ADU, one of the two is yours to live in and the other can be rented. An investor who does not live on the lot cannot use this route.

On size, the code reads: the ADU "shall not exceed eight hundred (800) square feet or fifty (50) percent of the gross floor area" of the main house, "up to a maximum of sixteen hundred (1,600) square feet." The cap is 1,200 square feet in the RE district. Ask the City how it applies the two limits to your house before you draw plans.

SB 15 in the San Antonio Code

UDC 35-398.03, "Small Lot Single-Family Development," was adopted by Ordinance No. 2025-09-04-0580 on September 4, 2025, three days after SB 15 took effect. On unplatted tracts of five acres or more zoned for single-family homes, the section says a development "is not required to have platted residential lots: (A) larger than three thousand (3,000) square feet in area; (B) wider than 30 feet; (C) deeper than seventy-five (75) feet."

Small lots of 4,000 square feet or less get the other SB 15 limits: setbacks of no more than 15 feet in front, 10 at the rear and 5 on the side; no more than one parking space per unit; no more than 30 percent open space; three stories allowed; and no bulk or wall articulation rules. Land within 3,000 feet of an airport or military base is not eligible, nor is a military clear zone or accident potential zone within 15,000 feet. The section names Lackland Air Force Base and Randolph Air Force Base.

Rezoning a Lot From R to RM

A lot zoned R-4 that you want to use for a fourplex needs a rezoning to RM-4, and that is a vote of City Council. For proposals made on or after September 1, 2025, HB 24 sets the protest rules in Local Government Code 211.0061. When the change allows more housing, a neighbor protest is valid only if owners of at least 60 percent of the land within 200 feet sign it, and a majority of all Council members can then approve the change. Owners of 20 percent of the land inside the change area can still force a three-fourths vote.

State law also sets the notice. Owners within 200 feet get mailed notice before the 10th day before the zoning commission hearing, and the Council hearing must be published in a newspaper and on the City's website before the 15th day before it. A rezoning that allows more housing is presumed valid if nobody sues to void it within 60 days after it takes effect. The HB 24 page has the details.

Outside the city limits, San Antonio's extraterritorial jurisdiction (ETJ) reaches five miles, the distance state law sets for a city of 100,000 people or more. In the ETJ the City applies its plat rules but, under Local Government Code 212.003, may not regulate a building's use, size or units per acre unless another law allows it. The UDC districts on this page apply inside the city.

What the Numbers Show

San Antonio had 1,548,422 people on July 1, 2025, by the Census Bureau's Vintage 2025 estimate, up from 1,434,625 in the 2020 Census.

Source: Census Bureau Building Permits Survey, South Region places, 2025 annual. Population from the Vintage 2025 city file.

San Antonio permitted 3,888 one-unit buildings and 8 buildings of three or four units in 2025, and the Census file shows 0 two-unit buildings. Houston permitted 1,054 two-unit buildings in the same year. The Census counts each building by the number of homes it holds.

Financing, Permits and Deed Restrictions

Loan rules can limit a plan the code allows. Fannie Mae's Selling Guide B2-3-04 says: "Only one ADU is permitted on the parcel of the primary one-unit dwelling. ADUs are not permitted with a two- to four-unit dwelling." A house plus one ADU fits that rule; an RM-4 fourplex plus an ADU does not. The financing page covers FHA and Fannie Mae rules.

For the building permit, Local Government Code 214.904 gives a Texas city 45 days to grant or deny an application, give written reasons it cannot yet decide, or agree on a deadline. If it misses the deadline it may not collect, and must refund, the permit fees. The UDC carries a fee schedule in its Appendix C; VanPlex did not read it for residential permits, so this page gives no fee amounts.

Recorded deed restrictions and HOA rules can forbid an ADU or a second home that the UDC allows, and Property Code 202.003 tells courts to read them broadly. Read the restrictions for your subdivision first.

Where to Go Next

Related on VanPlex: the SB 15 small lots page, the building codes page and the Texas multiplex guide overview.

Frequently Asked Questions

Official sources (checked October 2026)