Austin floor area calculator: how much can you build?
Which rule applies to you
This is the part that decides everything, and it happens before any arithmetic:
- One dwelling, lot 5,750 sq ft or larger — Subchapter F §2.1.
- One dwelling, lot 1,800–5,749 sq ft — §25-2-779(J), added by HOME Phase 2.
- Two or three dwellings — §25-2-773(E), added by HOME Phase 1. Minimum lot area 5,750 sq ft.
All three are geographically bounded. §25-2-773(E)(1) and §25-2-779(J)(1) each tie themselves to "the area described in Subsection 1.2.1 of Subchapter F" — the ring formed by Highway 183, Ben White Boulevard, Loop 360, Loop 1 and the Colorado River. Outside it, none of these caps apply.
The formulas
Every cap below is "the greater of a ratio or a flat square footage" — so small lots get a workable allowance and large lots scale.
- Subchapter F: greater of 0.4 × lot, or 2,300 sq ft. Equal at 5,750 sq ft.
- Small lot: greater of 0.55 × lot, or 1,650 sq ft — then capped at a 2,300 sq ft maximum unit size. Three bands: the floor governs below 3,000 sq ft, the ratio between there and 4,182 sq ft, and the unit cap above that.
- Two units: site total = greater of 0.55 × lot or 3,200 sq ft. Any one unit = greater of 0.4 × lot or 2,300 sq ft.
- Three units: site total = greater of 0.65 × lot or 4,350 sq ft. Any one unit = greater of 0.4 × lot or 2,300 sq ft. Any two units = greater of 0.55 × lot or 3,200 sq ft — rising to 0.65 or 4,350 sq ft if an existing dwelling unit is preserved.
| Lot size | Governing rule | One dwelling | Decided by |
|---|---|---|---|
| 1,800 sq ft | §25-2-779(J) | 1,650 sq ft | 1,650 sq ft floor |
| 2,500 sq ft | §25-2-779(J) | 1,650 sq ft | 1,650 sq ft floor |
| 3,000 sq ft | §25-2-779(J) | 1,650 sq ft | 0.55 ratio |
| 4,000 sq ft | §25-2-779(J) | 2,200 sq ft | 0.55 ratio |
| 4,182 sq ft | §25-2-779(J) | 2,300 sq ft | 0.55 ratio |
| 5,000 sq ft | §25-2-779(J) | 2,300 sq ft | 2,300 sq ft unit cap |
| 5,749 sq ft | §25-2-779(J) | 2,300 sq ft | 2,300 sq ft unit cap |
| 5,750 sq ft | Subchapter F §2.1 | 2,300 sq ft | break-even |
| 6,500 sq ft | Subchapter F §2.1 | 2,600 sq ft | 0.40 ratio |
| 7,000 sq ft | Subchapter F §2.1 | 2,800 sq ft | 0.40 ratio |
| 8,500 sq ft | Subchapter F §2.1 | 3,400 sq ft | 0.40 ratio |
| 11,000 sq ft | Subchapter F §2.1 | 4,400 sq ft | 0.40 ratio |
One dwelling, inside the Subchapter F boundary. The regime changes at 5,750 sq ft — note that the allowance does not jump there, because the small-lot unit cap and the Subchapter F floor are both 2,300 sq ft.
| Lot size | Units | Site total | Any one unit | Any two units |
|---|---|---|---|---|
| 5,750 sq ft | 2 units | 3,200 sq ft | 2,300 sq ft | — |
| 5,750 sq ft | 3 units | 4,350 sq ft | 2,300 sq ft | 3,200 sq ft |
| 7,000 sq ft | 2 units | 3,850 sq ft | 2,800 sq ft | — |
| 7,000 sq ft | 3 units | 4,550 sq ft | 2,800 sq ft | 3,850 sq ft |
| 8,500 sq ft | 2 units | 4,675 sq ft | 3,400 sq ft | — |
| 8,500 sq ft | 3 units | 5,525 sq ft | 3,400 sq ft | 4,675 sq ft |
| 11,000 sq ft | 2 units | 6,050 sq ft | 4,400 sq ft | — |
| 11,000 sq ft | 3 units | 7,150 sq ft | 4,400 sq ft | 6,050 sq ft |
Two and three units under §25-2-773(E). The per-unit caps are what stop a "three-unit" project being one large house with two token units attached.
Preserving an existing house raises the sub-cap
Since May 2025, §25-2-773(E)(4)(c) has two branches. On a three-unit project the limit on any two units combined is the greater of 0.55 FAR or 3,200 sq ft normally — but the greater of 0.65 FAR or 4,350 sq ft if an existing dwelling unit on the site is preserved under Subsection (F).
On a 7,000 sq ft lot that is 3,850 sq ft against 4,550 — seven hundred square feet for keeping a house that is already there. The site total is unchanged either way; what moves is how much of it you may concentrate in two of the three units.
Separately, the preserved square footage of a qualifying existing unit can be excluded from gross floor area altogether. That is a different provision with its own requirements, and it applies to houses built on or before 31 December 1960, or built from 1961 onward and at least 20 years old.
The regimes do not measure floor area the same way
This trips people up more than the ratios do. The same house can produce two different countable numbers depending on which rule governs it.
Subchapter F §3.3. Subchapter F §3.3. Counts enclosed space plus upper-storey roofed areas, mezzanines and lofts. Excludes ground-floor porches, qualifying attics and qualifying below-grade space, with parking carve-outs of up to 450 and 200 sq ft. Ceilings over 15 feet count twice.
§25-2-773 and §25-2-779. The total ENCLOSED area of all floors with a clear height of more than six feet, measured to the outside surface of the exterior walls. An enclosed garage counts. Unenclosed space — porches, carports — does not. There are no attic or parking exemptions here.
So moving from one dwelling to two does not simply raise your allowance from 0.4 to 0.55 — it also removes the attic and parking exemptions and starts counting your enclosed garage. The higher ratios were calibrated with that in mind.
Methodology and limits
The calculator applies the statutory caps and nothing else. It gives you a ceiling, not a design.
What it does not account for: setbacks and setback planes, building coverage, impervious cover, height, heritage trees, watershed regulations, or any neighbourhood plan (NP), neighbourhood conservation (NC) or historic area (HD) combining district. Under §25-2-1406(9) an NP district can modify the maximum FAR and maximum gross floor area outright — so if your lot is in one, the number here may simply be wrong for you.
On a typical Central Austin infill lot, floor area is rarely the first constraint you hit. Impervious cover usually runs out sooner. Treat this as one of several limits that all apply at once, and the binding one is whichever runs out first.
How we use a number like this
As a feasibility screen, not a design target. It tells us whether a lot is worth drawing before anyone spends money drawing it, and it tells us which regime we are designing under — which matters more than the number, because it changes what counts.
The moment a project looks like it wants two or three units, the whole calculation changes shape: more total floor area, but no attic exemption, no parking carve-outs, and a per-unit cap that shapes the massing. Better to know that at the sketch stage.
Common questions
Working out what you can build?
We build in Central Austin and deal with this code on every project. If you own a lot and want a straight answer about what fits on it, we're happy to take a look.
Talk to us about your lotWe 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.