Shed & fence lot
60 × 120 ft · 3 surfaces · limits 35% / 50%
- Lot coverage
- 24.8% PASS
- Impervious
- 29.0%
- Coverage left
- 732 sq ft
- New impervious
- 192 sq ft
Impervious surface and lot coverage calculator · no drawing needed
Impervious Surface and Lot Coverage Calculator Site Plan Worksheet
List the roofs, decks, pools and paving on your lot to get building lot coverage, impervious surface and the area you have left under each limit.
| # | Name | Width | Depth | or Area | Kind | Status | Counts |
|---|---|---|---|---|---|---|---|
| 1 | Coverage + impervious | ||||||
| 2 | Coverage + impervious | ||||||
| 3 | Impervious only |
A number in “or Area” overrides width × depth. *Gravel counts as impervious in some stormwater rules.
Worked examples
Four lots, already filled in
The same four plans as the site plan presets, as tables. Load one and change a row.
Deck & patio lot
50 × 100 ft · 4 surfaces · limits 35% / 50%
- Lot coverage
- 30.4% PASS
- Impervious
- 35.6%
- Coverage left
- 230 sq ft
- New impervious
- 380 sq ft
Detached ADU lot
75 × 140 ft · 5 surfaces · limits 40% / 50%
- Lot coverage
- 25.9% PASS
- Impervious
- 31.2%
- Coverage left
- 1,480 sq ft
- New impervious
- 974 sq ft
Metric council lot
18 × 32 m · 4 surfaces · limits 50% / 60%
- Lot coverage
- 32.8% PASS
- Impervious
- 41.8%
- Coverage left
- 99.0 m²
- New impervious
- 48.0 m²
What counts
Coverage vs impervious, surface by surface
Lot coverage limits building bulk; impervious limits protect drainage. The two lists overlap but are not the same, and many rows depend on your local code.
| Surface | Building lot coverage | Impervious surface |
|---|---|---|
| House, garage, ADU, shed (roofed) | Counts | Counts |
| Covered porch or carport | Counts in most codes | Counts |
| Attached or raised deck | Varies — often counts | Varies |
| In-ground pool | Usually not | Usually counts |
| Concrete or asphalt driveway, patio, walk | No | Counts |
| Pavers on sand or permeable pavers | No | Varies — some credit |
| Gravel drive or patio | No | Varies |
| Lawn, planting beds, mulch | No | No |
Stormwater rules care about impervious area because every square foot of roof or paving sends rain straight to the gutter instead of the soil. One inch of rain on 1,000 sq ft of roof is about 623 gallons, which is why towns cap impervious area or ask for a drywell once you pass a threshold.
When your code is silent on decks, gravel or pavers, ask the zoning office and note their answer on the application. Changing a row's kind here, or an element's toggles on the site plan, keeps your numbers consistent with that answer.
How it works
Coverage in four steps
Enter the lot
Type width and depth, or the lot area from your tax card, plus your zoning limits.
List each surface
One row per roof, deck, pool and paved area — by width × depth or by area.
Pick the kind
Building, deck, pool, paving or pervious decides which total each row adds to.
Read what is left
Coverage and impervious update with every keystroke, with the area left before each limit.
Use cases
When to run the numbers
- Before you design an addition
- Find out how many square feet of footprint the lot still allows before you pay for drawings.
- Choosing deck vs patio
- Switch one row between deck and paving to see which keeps you under both limits; then draw it on the deck and fence plot plan.
- Filling in a permit form
- Many applications ask for existing and proposed coverage. Copy the formula line, or let the site plan creator print it in the Pro coverage table.
- Shed or ADU feasibility
- Add the shed or ADU row to see the new coverage; the shed tool and ADU example carry it to a drawing.
- Checking the math
- The how-to guide and the scale guide explain where these numbers go on the sheet.
FAQ
Questions people ask before they submit
What is the difference between lot coverage and impervious surface?
Lot coverage counts building footprints (roofed area) to control bulk. Impervious surface adds every hard surface water cannot soak into — drives, patios, walks, usually pools — to control stormwater runoff.
Do I measure to the wall or the roof edge?
Most codes measure coverage at the outside face of the walls; some include roof overhangs over about 2 ft. When unsure, use the larger roof outline — it errs on the safe side.
Why does my pool count only as impervious?
An in-ground pool has no roof, so most codes leave it out of building coverage, but its water and deck shed rain like pavement. Change the row kind if your town treats it differently.
How do I find my coverage limit?
It is in your zoning district's dimensional table, often titled "maximum lot coverage" or "maximum building coverage", next to the setbacks. Residential limits of 25–40 % are common.
Can I enter areas instead of sizes?
Yes. A number in the Area column overrides width × depth, which helps for L-shaped houses or areas copied from an assessor card.
Same engine, other jobs
More site plan tools
Every tool shares the drawing engine, the units switch and the Pro export pack, so a plan started on one page opens the same way on another.
DIY Site Plan CreatorLot, setbacks, structures and live coverage on one sheet.Open tool →Plot Plan CreatorTrace a plat or aerial screenshot after a 2-point scale.Open tool →Shed Permit Site PlanShed size, distances to lot lines, accessory setbacks.Open tool →Deck & Fence Plot PlanDeck clearance, fence runs, heights and new impervious area.Open tool →HOA Site Plan ToolArchitectural review title block and packet checklist.Open tool →
Guides: How to draw a site plan for a building permit · Scales, sheet sizes and title blocks · Residential site plan examples.