Lot Coverage and Impervious Surface: How to Calculate Yours

Lot coverage and impervious surface are two different measurements of how much of your lot is "used up." Lot coverage (sometimes called building coverage) usually counts roofed buildings. Impervious surface counts those buildings plus driveways, patios, walkways and other hard surfaces that keep rain from soaking into the ground. Floor area ratio (FAR) is a third number some cities use: it compares the floor area inside the building to the lot size, not the footprint on the ground.

To calculate yours, measure each surface your city counts, add the square feet, divide by the lot area, and compare the percentage to the limit in your zoning district. A patio or shed can pass a building-coverage check and still fail an impervious-surface or stormwater rule—or the reverse. The definitions below come from published city and county codes; they do not match everywhere, so always use the worksheet your own permit counter publishes.

This guide is the calculation deep-dive. For patio permits and drainage triggers, see our concrete and paver patio guide. For drawing the lot on a scaled plan, see how to draw a building permit site plan. Last checked: November 6, 2026.

Three numbers cities track (and why they disagree)

Homeowners often treat "lot coverage" as one idea. Reviewers usually don't. Building coverage protects bulk and open yard. Impervious cover protects stormwater and neighbors downhill. FAR controls how large the building can feel relative to the lot, including upper floors. One project can pass one test and fail another.

Austin, Texas is a clear published example of the split. Its land development code defines building coverage as the area of a lot covered by buildings or roofed areas, and it excludes ground-level paving, landscaping, open recreational facilities, and incidental projecting eaves and balconies (Austin City Code § 25-1-21). Separately, Austin defines impervious cover as the total area of any surface that prevents infiltration into the ground—roads, parking, concrete, and buildings—and then lists what to include and exclude in the calculation (§ 25-1-23 and § 25-8-63).

Hillsborough, California splits the idea a different way. Its zoning chapter defines three residential coverages: structural (house, garage, carport and other accessory structures with more than one wall and a roof), hardscape (areas under a roof or covered by manufactured materials that are not already counted as structural), and softscape (everything else, such as lawn and plantings). It also defines an impervious surface as a constructed surface that prevents water from reaching the soil or that significantly increases runoff, and it allows certain pervious materials to be counted at half area when the applicant shows they infiltrate (Hillsborough Municipal Code § 17.32.060).

Those two cities do not use the same labels, and they do not count the same things. That is the point. Use the table as a comparison of published rules, not as a universal calculator.

Sources for the table: Austin § 25-1-21, § 25-1-23 and § 25-8-63; Hillsborough § 17.32.060; Montgomery County DEP, Rate and Charge Calculation (WQPC).

What counts as impervious vs pervious

Start with your city's own include/exclude list. If you only have a zoning glossary and no stormwater worksheet, treat every hard surface as a candidate until the counter tells you otherwise. The U.S. EPA notes that in urban and suburban areas, buildings, pavement and compacted landscapes keep rain and snowmelt from soaking in, which raises runoff volume and speed and carries pollutants into waterways (EPA, Urban Runoff / Low Impact Development). That is why many local rules count more than the house.

Use this checklist against your own ordinance. Where two published lists disagree, the table shows both rather than averaging them.

Do not assume a product marketed as permeable gets credit. Ask the permit counter which worksheet and which product detail they accept. Our patio guide walks through that product question for flatwork; this article stays on the math.

How to calculate yours: a worksheet you can copy

These steps work for either metric. The only change is which surfaces you put in the "counted" column.

1. Confirm the metric and the limit. Open your zoning district page (or call zoning). Write down the exact label: building coverage, lot coverage, structural coverage, impervious cover, impervious surface ratio, open-space minimum, or FAR. Copy the maximum percentage or ratio for your lot size and zone. Limits are local; this article does not invent a citywide number.
2. Get the lot area from a record. Use the survey, recorded plat, or assessor parcel data. Do not pace it off. If the code uses net lot area (excluding certain rights-of-way or constrained land), use that definition.
3. List every existing surface that might count. House, attached garage, detached garage, shed, covered porch, driveway, walkways, patio, parking pad, sport court, pool deck. For FAR, list floor areas by story instead.
4. Measure each one. Length × width for rectangles. Break odd shapes into rectangles. For building coverage, measure to the walls or columns your code names. For overhangs, check whether eaves are in or out—Austin's building-coverage definition excludes incidental projecting eaves; its impervious section has a separate residential-permit note about projecting elements.
5. Apply local credit or partial-count rules. Examples from the sources above: Austin's 50% deck rule; Hillsborough's 50% count for demonstrated pervious hardscape; Austin's porous-pavement exclusions when the Environmental Criteria Manual conditions are met.
6. Add the counted square feet. Call that total C.
7. Divide: coverage % = (C ÷ lot area) × 100. For FAR, divide gross floor area by gross site area. Round the way your worksheet says, or show two decimals.
8. Compare to the limit and to the room left. Room left = (limit % × lot area) − C. That leftover square footage is what you can still add under that metric.
9. Run the other metric if your city has both. A shed can fit under building coverage and still push impervious over the line. A patio can leave building coverage untouched and still trigger stormwater review.
10. Put the numbers on the site plan. Reviewers expect the arithmetic in a data box. See the site-plan guide for how to show it.

Montgomery County, Maryland adds a stormwater angle even when zoning coverage is not the issue. On lots under 15,000 square feet, additions that increase building lot coverage (the footprint of the main building or accessory structures such as attached garages, carports and sheds) by more than 400 square feet generally need a drainage plan, and any new detached accessory dwelling unit needs drainage review regardless of roof size (Montgomery County DPS, Guidelines for Control of Water Runoff on Small Lots (PDF)). Those guidelines size control measures for 1.5 inches of rain in 24 hours and state that 100 square feet of impervious area generates 12.5 cubic feet (about 94 gallons) of water under that design storm. That is runoff math, not a zoning coverage percentage—but it is often why a "small" addition gets held for drainage.

Worked examples (clearly labeled as made-up)

These are made-up examples. The lot sizes, building sizes and percentage limits below are invented to show the arithmetic. They are not the rule in Austin, Hillsborough, Montgomery County or anywhere else. Look up your real district limits before you design.

Example A — Building coverage only

Hypothetical lot: 50 ft × 100 ft = 5,000 sq ft. Hypothetical building-coverage limit: 35%. Existing house footprint 1,200 sq ft. Existing detached garage 400 sq ft. Proposed shed 120 sq ft.

In a building-coverage-only city, the driveway and patio would not appear in this table. In an impervious-cover city, they would.

Example B — Same lot under an impervious-surface limit

Keep the same 5,000 sq ft lot and the same buildings. Add a 600 sq ft driveway and a 300 sq ft concrete patio. Hypothetical impervious limit: 45%. Assume, for the example only, that the city counts buildings, driveway and patio at 100% and does not credit the patio as permeable.

Impervious % = 2,620 ÷ 5,000 = 52.4%. Against the example 45% cap, it fails by 2,620 − (0.45 × 5,000) = 2,620 − 2,250 = 370 sq ft. The shed alone is only 120 sq ft; the driveway and patio already used most of the budget. That is a common reason a "small" shed or patio expansion is denied even when building coverage still looks fine.

Example C — Partial deck credit

Hypothetical: the city uses Austin's published 50% rule for an uncovered wood deck with drainage spaces over a pervious surface. Deck is 12 ft × 16 ft = 192 sq ft. Counted impervious from the deck = 0.5 × 192 = 96 sq ft. If the same deck were a solid concrete patio, many worksheets would count all 192. The product and the detailing change the number; the marketing name on the box does not.

Example D — FAR vs coverage

Hypothetical 5,000 sq ft lot. Two-story house with 1,200 sq ft footprint and 2,200 sq ft of gross floor area. Building coverage = 1,200 ÷ 5,000 = 24%. FAR = 2,200 ÷ 5,000 = 0.44. A one-story 1,800 sq ft ranch on the same lot would show higher building coverage (36%) and lower FAR (0.36). If your district caps both, you have to clear both.

Why patios and sheds get denied on coverage math

From the published rules above, denials usually come from one of these mismatches—not from a mysterious extra percentage:

• You ran the wrong metric. You checked building coverage. The reviewer checked impervious cover or a drainage threshold. Austin's two definitions make that split explicit. Montgomery County's small-lot drainage trigger is based on added building footprint, not on whether zoning coverage still has room.
• Existing flatwork already used the budget. Example B is the pattern: the new shed is small, but the driveway and patio were never subtracted from anything when you "only looked at the house."
• Overhangs, columns or porches were measured wrong. Hillsborough's structural rule measures to exterior walls or supporting columns and has a limited porch/overhang exception. Austin's building-coverage definition excludes incidental projecting eaves. Measuring to the drip edge when the code wants the wall (or the reverse) swings the percentage.
• A permeable product got no credit. Austin's porous-pavement exclusion is conditional. Hillsborough's half-count needs a demonstration that water reaches the soil. Montgomery County still maps permeable pavement as impervious for the WQPC and handles relief through credits. Buying "permeable pavers" without the city's accepted detail is a frequent miss; the patio guide covers the product side.
• Gravel or compacted base was treated as green space. Montgomery County's WQPC page treats vehicle gravel as impervious. Compacted base under decorative rock often fails the "landscaping gravel over pervious ground" idea that some codes carve out.
• HOA limits are tighter than the city. An HOA can ban a patio your city would allow. That is a private covenant, not zoning. See HOA approval vs city building permit.

If the project fails setbacks rather than coverage, that is a different problem. Coverage is about area; setbacks are about distance to the lot line. Our zoning variance guide is for the distance problem. For sheds and garages as projects, see shed permit requirements and detached garage permits.

Decision flow when you are near the limit

Work top to bottom. Stop when you have a path.

1. Which published limit applies? Building/structural coverage only, impervious/hardscape only, both, FAR, open-space minimum, or a stormwater/drainage trigger tied to added footprint. Write the source section on your worksheet.
2. Does the existing lot already exceed it? If yes, new hardscape or roof area needs a reduction elsewhere, a credited practice your city accepts, or a formal relief process your code allows. Do not assume a variance exists for coverage the way it sometimes does for setbacks.
3. Will the new work pass building coverage but fail impervious (or the reverse)? Redesign for the stricter metric. Shrinking a patio helps impervious; it does nothing for building coverage. Shrinking a shed helps both when the shed is roofed.
4. Is stormwater review separate? On small Montgomery County lots, added building footprint over 400 square feet can require a drainage plan even when you are under a zoning percentage. Check your county's drainage or grading threshold before you call the project "exempt."
5. Can a credited surface replace a counted one? Only with the city's accepted detail. EPA describes low-impact practices—rain gardens, permeable pavements, rain barrels, vegetated roofs—as ways to keep more water on site. Local credit is what matters for the permit math.
6. Still short? Cut area, combine functions (one pad instead of two), or move work indoors. Then redraw the site plan and rerun both metrics before you resubmit.

Frequently asked questions

Is lot coverage the same as impervious surface?

No. In Austin's published definitions, building coverage is about buildings and roofed areas; impervious cover is about surfaces that block infiltration, including paving. Hillsborough separates structural coverage from hardscape. Some cities use "lot coverage" the way Austin uses building coverage; others use it more broadly. Read the local definition, not the everyday meaning of the words.

Do driveways count?

Toward impervious cover, usually yes—Austin § 25-8-63 lists driveways, and Montgomery County's WQPC includes them. Toward building or structural coverage, usually no, unless your code says otherwise. Always confirm.

Do decks count?

It depends. Austin counts an uncovered wood deck with drainage spaces over a pervious surface at 50% for impervious cover. Montgomery County's WQPC page lists wooden decks as not included. A roofed deck or screened porch is more likely to count as building coverage. Check both metrics.

What about a room addition?

A room addition adds building footprint and usually adds impervious roof area. It can also change FAR. Run all of the limits your district publishes, and see room addition permits for the broader permit path.

Can I fix a coverage denial by calling the patio "permeable"?

Only if your city accepts the product and the installation detail on its worksheet. Austin's porous-pavement exclusion is narrow. Hillsborough requires a demonstration of infiltration for its half-count. Montgomery County may still map permeable pavement as impervious for the stormwater charge while offering credits. Get the counter's rule in writing before you pour.

Bottom line

Calculate the metric your city actually enforces, then calculate the second metric if one exists. Building coverage cares about roofed footprints. Impervious cover cares about hard surfaces and runoff. FAR cares about floor area. Measure from records, apply the local include/exclude list without averaging conflicting city rules, and leave yourself a cushion for the next shed or patio. Put the arithmetic on the site plan, and treat permeable products as uncredited until the permit counter says otherwise.

When you are ready to file, start with how to apply for a building permit, confirm whether the work needs a permit at all with the Do I Need a Permit? checker, and find local portals through the building permit search by state.

Official sources

• Austin, TX — § 25-1-21 Definitions (building coverage; floor area ratio; Corpus mirror of city code)
• Austin, TX — § 25-1-23 Impervious Cover Measurement (Corpus mirror of city code)
• Austin, TX — § 25-8-63 Impervious Cover Calculations (Corpus mirror of city code)
• Hillsborough, CA — § 17.32.060 Residential lot coverage (structural, hardscape, softscape)
• Montgomery County, MD DEP — Water Quality Protection Charge rate and impervious-surface list
• Montgomery County, MD DPS — Guidelines for Control of Water Runoff on Small Lots (PDF)
• U.S. EPA — Urban Runoff / Low Impact Development

Last checked: November 6, 2026. Rules are local and change. This page explains published definitions and arithmetic; it is not a substitute for your zoning worksheet, stormwater manual or a design professional. Hypothetical examples are labeled as made-up and are not limits for any city.

Edited by Ahmed Khalifa, founder of PermitCove. Sources are linked inline; see our Editorial Policy.