What can I build on my lot in Austin?
Nobody asks this question in code order
An owner standing in their own back yard asks one question: what can I build here? The code has no page for that. It has a page for floor area, a page for impervious cover, a page for setbacks, a page for height, and so on — each written as though you already knew to look it up.
So this page is a map rather than a rulebook. It names each constraint, says what kind of limit it is and why it tends to bite, and sends you to the page that owns the figures. Deliberately, it repeats none of them. A number copied onto two pages goes stale on one of them the day the other is corrected, and you would have no way of telling which copy you were reading. Every figure on this hub lives on exactly one page, next to the section it came from and the date a human last checked it.
What this page adds instead is the part no single constraint page can tell you: the order they bind in, and which ones reach your lot at all.
| Constraint | What shape it takes | Where the figures live |
|---|---|---|
| Unit count | A count — but reaching it depends on a use classification, which in turn depends on lot area | Units & ADUs |
| Zoning district | The base schedule. Every other rule on this list is either an entry in it or an exception to it | SF-3 · all districts |
| Floor area | A ratio of floor area to lot area — with its own definition of which floors count | Floor area ratio · what counts |
| Impervious cover | A percentage of the lot you may cover with anything water cannot pass through — driveway included | Impervious cover |
| Setbacks | Distances from each lot line inside which nothing may be built. They shape the footprint before area ever does | Setbacks |
| Height | A vertical limit, plus a rule about the plane it is measured from — the second half decides more cases than the first | Height |
| Trees | A permit trigger keyed to trunk diameter, and a root zone around each protected tree that constrains where you may excavate | Trees |
| Watershed | A geographic layer that can add rules the zoning schedule never mentions, and can put part of the lot out of reach entirely | Watersheds · water quality zone |
The eight constraints that decide most Austin residential projects, by shape rather than by figure. Each links to the page that carries its numbers, its sections and its own verification date. This is a map of the common ones, not a complete list — see below.
They are layers, and the layers have a stated order
The most common way to get this wrong is to read your zoning district's schedule, write down the numbers, and design to them. That works only if nothing overrides them — and two of the biggest changes in recent Austin code exist precisely to override them.
Your zoning district is the base layer. It is where the schedule of limits lives, and if nothing else applies, it governs. Start at SF-3 if that is your zoning, or at the district index if it is not.
Then the use you choose can supersede it. §25-2-773 — the section governing duplex, two-unit and three-unit residential uses — opens by saying that to the extent of conflict, this section supersedes the base zoning district regulations. §25-2-779, the small lot single-family section, says the same thing about itself. Two different projects on the same lot, under the same zoning, can therefore be governed by different numbers. Which set applies to you is a consequence of what you decide to build.
Then the overlays. Subchapter F, the "McMansion" standards, is a separate rulebook with its own applicability tests, and it does not reach every project — HOME Phase 1 struck Duplex Residential and Two-Family Residential from its applicability use list, and HOME Phase 2 put small lots zoned SF-3 or more restrictive outside it except for one gross floor area provision. Whether it reaches your project is a question with its own page: does Subchapter F apply to my lot?
Then the geography. Your watershed, the floodplain, and any overlay or neighborhood plan combining district your lot sits inside are layers nobody chooses. They are also the ones most often discovered late.
Decide the unit count first — it changes almost everything below it
If you take one thing from this page, take this: the number of dwelling units you are building is not one constraint among eight. It is the switch that selects which version of the other seven you are subject to. Setbacks, floor area, impervious cover, height and whether Subchapter F reaches you at all can each change when you go from one unit to two.
And here is the trap, because it has caught people twice on this hub alone. A district maximum is a ceiling, not an entitlement. §25-2-492(D) sets a maximum number of dwelling units per lot for your district — but reaching that maximum requires building a duplex, two-unit or three-unit use, and §25-2-773(B)(1) sets a minimum lot area for those uses. A lot below that floor has no multi-unit option at all, whatever the district ceiling says. Reading the ceiling on its own is how an owner plans a project the lot cannot legally hold.
Both figures — the ceiling and the floor that gates it — are on how many units you can build, next to the sections they come from. Check your lot area against the floor before you plan around the ceiling. If your lot falls below it, see small lots, which is the regime that then applies.
Which of these rules actually reach a lot that already exists
A great deal of Austin's code is written about land being divided rather than about a house being built. If your lot already exists and is already platted, some rules that look alarming do not reach you — and some that look purely procedural very much do. Telling them apart is a matter of reading how each provision is phrased.
The hinge is a definition. §25-1-21 defines DEVELOPMENT to include the construction or reconstruction of a building, the placement of a structure on land, excavation, grading or filling, and the removal of vegetation. Building a house is development. So a provision written in terms of "development" reaches you on an existing lot, while one written in terms of what a lot may contain, or what a plat may be approved with, was settled when the lot was created.
- "Development is prohibited in…" — reaches a build on your platted lot.
- "A lot may not include…" — settled at platting. Not a live constraint on your build.
- "A plat may not be approved unless…" — subdivision only. See subdivision.
- An express carve-out — read the carve-out, because it will not follow the pattern.
This is a way of reading provisions, not a rule the code announces about itself. It is a starting point that tells you which sections are worth reading closely, and it does not substitute for reading them.
The impervious cover requirements of this Subchapter do not restrict impervious cover on a single-family or duplex lot but apply to the subdivision as a whole.
Being exempt from a review is not being exempt from the rule
The last piece of the map is the one people most often mistake for good news.
§25-5-1 requires a site plan to be approved and released before development, except as provided in §25-5-2. And §25-5-2(B)(1) exempts the construction or alteration of a small number of residential units — the exact threshold and its conditions are on site plan — provided the lot contains no more than that number including existing units, and the proposed improvement is not in the 100 year flood plain, or the director determines it will have an insignificant effect on the waterway.
Two things follow, and both matter more than the exemption itself.
The exemption is conditional, and the condition is geographic. A lot with floodplain on it can lose the exemption entirely — which changes your review track, your drawings and your timeline. Check the floodplain maps page early, not late.
And where the exemption does apply, it changes the vehicle, not the rule. Nothing about being outside site plan review makes a setback, a tree or a floodplain requirement stop applying — it changes which application they are checked on. §25-7-33(D)(4) proves the point flatly: a residential building permit must still depict a 100-year floodplain, a FEMA floodplain and a drainage easement. The requirement followed you onto the smaller application. See permits for what that application involves.
What we work out first, and in what order
The unit count, before anything is drawn. Not because more units are always better, but because the answer selects which rulebook the rest of the project lives under. Drawing to the wrong one and correcting later costs more than deciding it first — and the decision turns on lot area, which is the cheapest fact on this list to establish.
Then the geography, because it is the only part nobody can change. Watershed, floodplain, overlays, and where the protected trees stand. Every other constraint on this page is arithmetic you can design against. These are facts about the parcel, and they are the ones that turn a scheme into a different scheme rather than a smaller one.
Then we find which single constraint binds first. This is the part clients are most often surprised by: on any given lot, seven of the eight have room in them and one does not. Floor area and impervious cover trade against each other, setbacks and height decide whether the floor area is usable at all, and which one runs out first is a property of the lot rather than of the code. There is no general answer, which is why this page does not offer one.
Then we draw the envelope and see what fits. A buildable envelope drawn against the real constraints is worth more than any table, including ours — and it is usually the point at which an owner discovers the project they wanted is either comfortably available or was never possible, rather than somewhere expensive in between.
Common questions
Buying a lot in Austin: what to check before you sign — A short, boring list of checks that has saved more Austin projects than any design decision we have made. The list is our judgment. Every constraint on it is code, and we link to the code.
Judgment, not code. Those pages carry no verification date and cite no ordinance — where they depend on a rule, they link back to these pages.
Want to know what actually fits on your lot?
We do this research ourselves on every project we take on — zoning, floodplain, watershed, trees, whether it is a legal lot. Send us an address and we will run it and tell you what we find. It works the same whether you own a vacant lot, a house you are thinking of adding a unit to, or a house you are thinking of replacing. No charge and no strings, and if it is a bad lot we will say so.
Send us your addressWe are homebuilders, not the City of Austin. This page explains how we read the code in practice — it is not legal advice and it is not an official determination. Always confirm with Austin Development Services before you rely on it for a project. The authoritative text is the Austin Land Development Code; permitting questions go to Austin Development Services Department.