Austin secondary apartment special use: the pre-HOME second dwelling

Last verified August 6, 2026 against the Austin Land Development Code · Kimbrough Gray · KiliGray Homes

A secondary apartment must be in a structure other than the principal one, at least 10 feet to the rear or side or above a detached garage, no more than 30 feet and two stories, and no larger than 1,100 sq ft or 0.15 FAR — whichever is smaller.

Applies to: Secondary apartment special uses in an NP combining district whose ordinance prescribes the use. It cannot be combined with a cottage or urban home special use.

You probably do not need this. Since HOME, two- and three-unit residential are permitted uses outright in SF-1, SF-2 and SF-3 — no special use, no NP ordinance, and none of the size limits below. The secondary apartment is the older mechanism, it survives only inside NP combining districts, and it is substantially more restrictive. It is worth understanding mainly because older advice still points at it.

The size limit is not 1,100 square feet

This is the number most often quoted and most often quoted wrongly. §25-2-1463(C)(5)(a) caps a secondary apartment at:

1,100 total square feet or a floor-to-area ratio of 0.15 — whichever is smaller.

Those two only coincide on a lot of 7,333 sq ft. Below that, the FAR binds first:

on a 5,750 sq ft SF-3 lot — 0.15 gives 863 sq ft, not 1,100;
on a 7,000 sq ft lot — 1,050 sq ft;
only at 7,333 sq ft and above does the flat 1,100 cap take over.

So on most Central Austin lots the real allowance is smaller than the headline figure, and a scheme designed to 1,100 sq ft will not fit.

A second cap sits inside it: no more than 550 sq ft on the second story, if there is one.

It has to be a separate building

§25-2-1463 is unusually prescriptive about where the thing goes. A secondary apartment:

must be in a structure other than the principal structure — the article says so twice, in (B) and again in (C)(1);
must be at least 10 feet to the rear or side of the principal structure, or above a detached garage;
may be connected by a covered walkway — but that is the only permitted physical link;
may not exceed 30 feet in height and is limited to two stories.

An internal conversion — carving a second dwelling out of the existing house — is not a secondary apartment. Nor is an attached addition. The garage-top option is the one that makes this work on a narrow lot.

Not combinable with a cottage or urban home. §25-2-1463(A) is explicit. If your NP ordinance permits several special uses, you are choosing between them on a given lot, not stacking them.

Site-wide cover limits come down with it

Two whole-site caps apply where a secondary apartment is built:

impervious cover for the site may not exceed 45 percent; and
building cover may not exceed 40 percent.

On SF-3 land those match the base district figures, so nothing is lost. On a district that would otherwise allow more, they bind — and note they are site limits, counting the principal house, the secondary apartment, the driveway and everything else together.

One place the secondary apartment is favored

It gets a specific mention in §25-2-1406(5), the provision letting an NP ordinance cut minimum lot dimensions. That relief is available for a single-family residential use on a lot platted on or before 15 August 2024 — or for a secondary apartment special use on an existing legal lot, with no platting date attached.

Where an NP ordinance takes it up, that means minimum lot area as low as 2,500 sq ft and width as low as 25 feet. It is the one respect in which this older mechanism is treated more generously than the general rule.

When it still matters

Rarely as a design choice, often as a title question. Plenty of Austin lots already carry a secondary apartment approved under this article years ago, and understanding the constraints it was built under — separate structure, 30 feet, two stories, the FAR cap — explains why an existing garage apartment is the size and shape it is.

For new work, the HOME route is almost always better. Three permitted units with §25-2-773 floor area is a different proposition from one 860 sq ft secondary dwelling in a detached building.

The exception is where an NP ordinance has taken up the §25-2-1406(5) lot-size relief. There the secondary apartment route can unlock a lot that is otherwise too small to work with, and that is worth checking against the specific neighborhood ordinance.

Common questions

How big can a secondary apartment be in Austin?

1,100 sq ft or a 0.15 floor-to-area ratio, whichever is smaller — so on a 5,750 sq ft lot the real limit is 863 sq ft. No more than 550 sq ft may be on the second story.

Can a secondary apartment be inside my house in Austin?

No. §25-2-1463 requires it to be in a structure other than the principal structure, at least 10 feet to the rear or side, or above a detached garage. A covered walkway may connect them.

Is a secondary apartment the same as an ADU in Austin?

No. Austin no longer has a general ADU regime — HOME made two- and three-unit residential permitted uses outright. The secondary apartment is an older special use that survives only inside neighborhood plan combining districts and is more restrictive.

How tall can an Austin secondary apartment be?

30 feet, and no more than two stories.

Can I have a secondary apartment and a cottage on the same lot in Austin?

No. §25-2-1463(A) provides that a secondary apartment is not permitted in combination with a cottage or urban home special use.

What impervious cover applies with a secondary apartment in Austin?

45% for the site, with building cover capped at 40%. Both are whole-site limits counting the principal house and everything else.

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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.

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We 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.