Jessica Martinsen · Austin Real Estate
ADUs and second units in Austin: what the HOME rules actually let you build
Austin's HOME ordinance in practice: units per lot, the constraints that really bind, and what zoning allows versus what your deed restricts.
Austin rewrote the math on single-family lots. The HOME ordinance - passed in phases starting December 2023 - allows up to three housing units on lots that previously held one, and its second phase cut the minimum lot size for a single unit to roughly 1,800 square feet, opening the door to smaller-lot subdivision. That is the headline; the buildable reality on any specific lot is set by a stack of quieter constraints, and reading them in the right order saves months.
Three site rules do most of the constraining. Impervious cover - the percentage of the lot that can be roof, driveway, and patio - typically caps around 45% in Austin's single-family zones, and an existing house with a big slab and circular drive may have already spent the budget an ADU needs. In the central neighborhoods covered by the McMansion rules, floor-to-area ratio limits total built square footage across all structures. And Austin's tree ordinance is not a suggestion: protected trees from 19 inches in diameter need a permit to remove, heritage species at 24 inches are close to untouchable, and a mature live oak in the backyard will design your ADU's footprint for you.
City rules are only half the answer, and the less binding half. Deed restrictions and HOA covenants are private contracts that survive every zoning reform - a 1970s restriction reading 'one single-family dwelling per lot' still blocks a second unit no matter what the ordinance now allows, and enforcement runs through your neighbors, not the permit office. Before designing anything, pull the recorded restrictions for the subdivision from the county clerk and read them personally. This single step separates lots where HOME is real from lots where it is a headline.
The practical build questions are the ordinary ones, priced for Austin. Utilities: an ADU can usually share the main house's water and electric service, but a separate electric meter is worth pricing if you ever want a clean tenant arrangement. Parking minimums were largely eliminated citywide in 2023, which removed a historic ADU killer, though your own tenants still need somewhere real to park. And if the plan is short-term rental income, check Austin's STR licensing rules before underwriting: the license categories and their availability are their own regime, they change, and an ADU business plan built on an unavailable license type is a garden office.
Multigenerational buyers should also look at the inventory that already exists. Casita floor plans, garage apartments in the older central neighborhoods, and homes with true second primary suites often price close to ordinary comparables and skip the two-year design-permit-build cycle entirely. Run both paths - buy-and-build versus buy-it-built - with honest numbers on construction pricing and your own tolerance for managing a project, and remember the homestead exemption covers the whole property including a backyard unit on your primary residence, while the rental income it produces is ordinary taxable income.
Common questions
- How many units does Austin's HOME ordinance allow on a single-family lot?
- Up to three on qualifying lots, with the second phase reducing the minimum lot size for a single unit to roughly 1,800 square feet. What a specific lot supports depends on impervious cover, FAR in McMansion areas, trees, and utilities - verify against the current code, which has been amended repeatedly.
- Can my HOA or deed restrictions block an ADU that zoning allows?
- Yes. Private covenants outrank zoning liberalization in practice - a one-dwelling deed restriction still binds. Pull the recorded restrictions from the county clerk before designing anything.
- What usually kills ADU projects in Austin?
- Impervious-cover budgets already spent by the existing house, protected trees dictating the buildable envelope, and deed restrictions discovered late. All three are checkable before you buy the lot.
- Do I need extra parking for an ADU?
- Austin eliminated most minimum parking requirements citywide in 2023, so usually no as a code matter. Practical parking for tenants and resale is a separate question your street answers.
- Can I rent an ADU on Airbnb?
- Only within Austin's short-term rental licensing regime, which restricts license types and availability and continues to change. Confirm the current rules for your situation before underwriting STR income; long-term rental faces no such licensing.
This page is a planning guide, not a market report. Confirm current prices, taxes, insurance and property facts against authoritative sources before acting on them.