What a Zilker Lot Actually Supports: Cover, FAR, and the Trees
Developer Intelligence · August 2026
The short answer: two numbers decide what fits on a standard Zilker lot, and they measure different things. Impervious cover is capped at 45 percent of the lot — that's everything rain can't pass through. Floor-to-area ratio is capped at 40 percent, and that's the conditioned space itself. Subchapter F shapes the massing on top of both, and the tree ordinance shapes where any of it can sit. The square footage builders find beyond those caps comes almost entirely from exemptions: attics, basements, detached parking and carports.
Owners walk into a first design meeting with a square footage target. The site has its own opinion. The gap between those two things is where Zilker projects get expensive, and it closes fastest when you understand which number is binding before you hire anyone. On most central Austin lots, FAR runs out before impervious cover does — but not always, and knowing which constraint you're actually fighting changes the entire design approach.
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45% Impervious Cover Of lot area, SF-2 and SF-3 | 40% Floor-to-Area Ratio Conditioned space — or 2,300 SF, whichever is greater | EXEMPT Attics & Basements Where the extra square footage comes from | 19" / 24" Tree Thresholds Protected, and heritage species |
The Two Numbers, and What Each One Actually Measures
Impervious cover, 45 percent. Everything water can't pass through: house footprint, driveway, walkways, patios, pool coping and decking, retaining walls, AC pads. This is the number most owners have heard of and the one they usually assume is binding.
Floor-to-area ratio, 40 percent. Conditioned floor area across all stories divided by lot size, capped at 40 percent of the lot or 2,300 square feet, whichever is larger. On any lot above 5,750 square feet, the 40 percent figure governs. In practical terms this is your conditioned space — the actual house — and it's the constraint that determines how big the home can be.
The two are easy to conflate and shouldn't be. Impervious cover is a horizontal measurement of the whole site: it counts your driveway, your patio and your pool, none of which are house. FAR is a volumetric measurement of the building: it counts your second floor, which uses no additional ground at all. A project can be comfortably inside one cap and against the wall on the other, and the design response is completely different depending on which.
Run It on a Real Lot
Take a 7,500 square foot Zilker lot, SF-3, inside the Subchapter F boundary.
Limit | Calculation | What It Gives You |
|---|---|---|
Impervious cover | 7,500 × 45% | 3,375 SF for house footprint, drive, walks, patio and pool combined |
FAR | 7,500 × 40% | 3,000 SF of conditioned space across all floors — before exemptions |
Now spend it. A 1,700 square foot ground floor, a 600 square foot driveway, 350 square feet of walkways and patio, and a pool with decking at 600 puts you at 3,250 of impervious cover — inside the cap with about 125 square feet of margin, which is nothing. Meanwhile FAR gives you 3,000 square feet of conditioned space: 1,700 down, 1,300 up.
That's a 3,000 square foot house on a 7,500 square foot lot, and the pool nearly broke the impervious budget. Which is exactly why so many Zilker builds either skip the pool or go looking for exemptions.
Know which number is binding. On a wide, shallow lot with a long driveway, impervious cover usually runs out first, and the answer is permeable materials and a shorter drive. On a compact lot with street parking, FAR runs out first, and the answer is exemptions. Designing against the wrong constraint wastes an entire schematic round.
Subchapter F Shapes What's Left
Subchapter F — the McMansion Ordinance — applies to most single-family lots in the urban core, and for central Austin projects it controls the design more than the base zoning numbers do. Beyond the 40 percent FAR cap it imposes a tent-shaped building envelope: a plane starting 15 feet above average grade at the side setback line and sloping inward at 45 degrees. It also requires articulation on long side walls and restricts garage-dominated street frontages.
The tent is why second floors in this part of town step back, why so many roofs are gabled rather than flat, and why a boxy two-story that would be fine in a suburb doesn't work here. It's also worth noting what HOME changed: Subchapter F now applies only to lots with a single dwelling. Duplex, two-unit and three-unit projects fall under different standards with substantially higher FAR — 55 percent for two units, 65 percent for three — which is a genuine strategic consideration on a larger lot.
Where Builders Find the Extra Square Footage
Gross floor area has exemptions, and experienced central Austin builders design around them from the first sketch. These are the ones that matter.
Exemption | How It Works |
|---|---|
Attic | Attic volume meeting the code's height and access conditions is excluded from gross floor area. This is the single reason so many central Austin houses have steep gabled roofs with generous attic volume that looks decorative from the street — it isn't. It's square footage the FAR calculation doesn't see. |
Basement | Below-grade space is excluded where the habitable portion sits below natural grade and natural grade surrounds a qualifying share of the perimeter walls. On Zilker's sloped lots this is the highest-leverage exemption available — the topography that costs you money on the foundation can hand you a full floor for free on the FAR sheet. |
Detached parking, 450 SF | Up to 450 square feet of a detached rear parking structure separated from the house, or connected only by a breezeway open on two sides. One per site. |
Attached parking, 200 SF | Up to 200 square feet of attached parking used to satisfy the minimum parking requirement. Can stack with the 450 exemption under the right conditions. |
Ground-floor porch | Exempt where the porch isn't accessible by car and isn't attached to a driveway, capped at 200 square feet if it sits below habitable space or a balcony. |
Unenclosed carport | Excluded from FAR and gross floor area where it meets the open-siding requirements — and it treats your cover budget more kindly than an enclosed garage does. See below. |
Exemption criteria are specific and technical, and small design decisions determine whether a space qualifies. Confirm applicability for your project with your architect and the City before relying on any of it.
Carport or Garage
This is the trade that comes up on nearly every constrained central Austin lot, and it's usually decided by arithmetic rather than preference.
An enclosed garage is a building. It consumes conditioned floor area beyond the parking exemptions, and it counts toward impervious cover. An unenclosed carport meeting the open-siding requirements drops out of the FAR and gross floor area calculation entirely, and it's treated more favorably on the cover side.
The practical consequence: on a lot where FAR is binding, converting a two-car garage to a carport can free up several hundred square feet of house. That's a bedroom. Owners who arrive wanting a garage and leave with a carport usually make the trade willingly once they see what it buys — and the design language of central Austin has adapted around it, which is why carports here read as intentional rather than cheap.
Then the Trees Decide Where It Goes
Cover and FAR tell you how much. The tree ordinance tells you where. Austin regulates trees in tiers, and Zilker's mature oak canopy means most lots have at least one that matters.
Tier | Size & Species | What It Means |
|---|---|---|
Unregulated | Under 19 inches diameter | No tree review application needed on residential property. |
Protected | 19 inches or more, any species | Removal or significant impact requires a Tree Ordinance Review Application, reviewed alongside your building permit. No more than 25 percent of canopy removed without a permit. |
Heritage | 24 inches or more, listed species — live oak, pecan, Texas ash, bald cypress, cedar elm and several oaks | Removal requires a variance, granted rarely and generally only for dead, diseased or hazardous trees. |
The constraint that actually reshapes plans isn't removal — it's the critical root zone. The CRZ extends roughly one foot of radius per inch of trunk diameter, so a 30-inch live oak carries a 30-foot protected radius: a 60-foot circle where excavation, grading, equipment staging and material storage are all regulated. On a 60-foot-wide lot, one well-placed oak can consume the buildable width without ever being touched.
Tree survey before design, every time. An architect drawing a footprint before the survey exists is drawing something that may need to be thrown out.
Considering a Zilker lot and want to know what it will actually support?
Lot area, zoning, tree survey and a cover-and-FAR budget give you a real answer in days, not months — and all of it is knowable before you go under contract. The Davis Agency works with the builders, architects and consultants doing this every week across 78704.
One Caveat Worth Checking
The numbers above govern standard SF-2 and SF-3 lots, which covers the large majority of Zilker. A minority of properties in and around the neighborhood carry additional environmental overlays — proximity to Barton Creek, a Critical Water Quality Zone, or the Barton Springs Zone, where the Save Our Springs Ordinance imposes far tighter impervious cover limits and permits no variances at all.
Those situations are the exception rather than the rule here, but the downside of missing one is severe. A civil engineer or land use consultant can confirm watershed classification and overlay status on a specific parcel in about a day. Do that before you underwrite, not after.
Frequently Asked Questions
What is the impervious cover limit in Austin?
For SF-2 and SF-3 single-family zoning, which covers most of central Austin including Zilker, impervious cover is capped at 45 percent of lot area. It includes the house footprint, driveway, walkways, patios, pool decking and retaining walls — everything water cannot pass through. It is a separate limit from floor-to-area ratio, which caps the conditioned space of the house itself at 40 percent. Properties inside environmentally sensitive overlays such as the Barton Springs Zone are subject to substantially tighter limits.
What is the FAR limit for a house in central Austin?
Subchapter F, the McMansion Ordinance, caps floor-to-area ratio at 40 percent of lot area, or 2,300 square feet, whichever is greater. On any lot larger than 5,750 square feet the 40 percent figure governs. Subchapter F now applies only to lots with a single dwelling; two-unit and three-unit projects fall under different standards allowing 55 percent and 65 percent respectively.
How do builders get more square footage under Austin's FAR limit?
Through gross floor area exemptions. Qualifying attic volume and below-grade basement space are excluded from the calculation, which is why steep gabled roofs and basements are common in central Austin new construction. Up to 450 square feet of detached rear parking and 200 square feet of attached parking can also be exempted, and unenclosed carports meeting open-siding requirements are excluded from gross floor area entirely.
Why do so many Austin custom homes have carports instead of garages?
Because the arithmetic favors it on constrained lots. An enclosed garage consumes gross floor area and counts fully toward building and impervious cover. An unenclosed carport that meets the open-siding requirements drops out of the FAR and gross floor area calculation and is treated more favorably on cover. On a lot where FAR is the binding constraint, that trade can convert into several hundred additional square feet of house.
Related Reading from The Davis Agency
→ HOME Phase 2 Math in 78704: When a Lot Split Pencils and When It Doesn't
→ Who's Actually Building in Barton Hills Right Now
→ Fixed Price vs. Cost-Plus: How 78704 Builder Contracts Actually Work
Evaluating a Zilker Lot?
The cover-and-FAR budget, the tree survey and the zoning confirmation are all answerable before you're committed to anything. The Davis Agency can help you get them answered and connect you with the builders and consultants who work these lots regularly.
Start the ConversationCall (512) 608-8811
Derrik Davis · Broker/Owner, The Davis Agency · CLHMS Certified · TREC License #558841 · Serving 78704 and the greater Austin luxury market since 2006. This article is general market commentary and is not legal or engineering advice. Confirm zoning, cover limits, FAR exemptions and tree status for any specific property with qualified professionals before making a development decision.
