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Home addition cost calculator
A 20 × 20 ft room addition costs about $95,165 mid-range — $238 a square foot with plans, permit and contingency. A bathroom addition is about $54,356, a primary suite $122,305, an attached two-car garage $67,044 and a 900 sq ft second story $236,802. It is priced trade by trade, because the tie-in, the plumbing and the heating cost the same on a small addition as on a big one.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-28 · Nothing you type leaves your browser
Home addition cost estimator
The addition, its size and its finish
Priced trade by trade, not by an average per square foot: the foundation, the shell and the interior grow with the size; the tie-in to the house, the plumbing and the heating do not.
One room on the ground floor, on its own foundation, opened into the house through an existing wall.
Along the house.
400 sq ft.
Thickened edges down past the frost line and a slab poured in one go.
Matching the house: better windows.
Here: the house ducts extended.
Used only to pick a regional labour rate. It stays in your browser.
Room, 400 sq ft, mid-range
$95,165
$238 a square foot · $68,157–$133,826
10–16 weeks of building
After the plans and the permit, which take one to three months of their own before anyone digs.
Why the square foot moves
10% does not grow with the size.
Opening the house, tying the roofs, the plumbing and the heating cost much the same for a small addition as for a big one. Double the size and the price per square foot falls.
Every kind of addition
At a typical size, mid-range, on the foundation chosen above.
What the builder's price is made of
- Foundation — slab on grade
Thickened edges down past the frost line and a slab poured in one go. The cheapest, and the floor ends up a step below a house on a crawl space.$5,600 - Shell, 400 sq ft
Framing, sheathing, the roof, windows and an exterior door, siding and insulation.$38,000 - Interior — mid-range
Matching the house: better windows, siding to match, hardwood or tile, solid doors and a few recessed lights.$28,000 - Tie-in to the house
Opening the wall with a header, tying the roofs together and matching the siding and shingles.$6,500 - Duct branch from the existing system
Supply runs, boots and registers, with the permit.$1,204 - Plans, engineering and permit
About 10% of the building cost.$7,930 - Contingency
10% for what opening the house turns up.$7,930
What it costs
National average. Room of 400 sq ft, mid-range, built by a general contractor, with plans, permit and a 10% contingency. Not included: furniture, landscaping, a larger electrical service, septic changes or rent while the house is open.
The bathroom rate is the bathroom remodel page’s; the heating from mini-split and ductwork; the frost depth your footings go to, in the table by state.
Worth knowing
- $238 a square foot, and the figure is not a rate: 10% of the building cost here — $7,704 — is opening the house wall and tying the roofs together, the heating and cooling, which cost much the same on a small addition as on a big one. That is why the square foot of a small addition is dear and that of a big one cheap.
- A room anyone may sleep in needs an emergency escape window — at least 5.7 sq ft of clear opening, the sill no higher than 44 in — and habitable rooms need a ceiling of at least 7 ft. Both are drawn in at the plan stage and cost nothing then; they are expensive to add after the inspector finds them missing.
- Slab on grade: $5,600. The footings go down past your frost line whatever the foundation, and the new floor has to meet the old one — a slab against a house on a crawl space ends a step down, which is why the foundation usually follows the house rather than the budget.
- Extending the existing ducts is $1,204 if the furnace or heat pump has capacity to spare. Many do not: an addition of 400 sq ft adds roughly 10,000 BTU of load, and a system already sized to the house leaves the new room cold. A mini-split sidesteps that for about $3,185.
- Plans, engineering, a survey and the permit are about 10% on top ($7,930), and 10% is held back for what opening the house wall turns up ($7,930). Setbacks and lot coverage decide how big the addition may be before any of this is spent — check them first.
On this page — 5 sections
Trade by trade, not a rate
The short version
- 20 × 20 room, mid-range: $95,165. $238 a square foot.
- Bathroom addition: $54,356. Small, and all plumbing.
- Second story, 900 sq ft: $236,802.
- Attached two-car garage: $67,044. The cheapest square foot.
An addition is a small house joined to a big one, and it is priced the way a builder prices it: by the trade. The foundation, the shell and the interior grow with the floor area — about $95 a square foot for the shell at mid-range. Opening the house wall and tying the roofs together (about $6,500), bringing plumbing out, and heating the new space cost much the same whatever the size. Plans, engineering and the permit add about 10%, and a 10% contingency covers what the open wall turns up.
addition = (foundation + shell + interior + tie-in + plumbing + heating) × (1 + design & permit + contingency)
$95,165
20 × 20 room
$238/sq ft
$54,356
Bathroom
8 × 10 ft
$122,305
Primary suite
16 × 24 ft
$236,802
Second story
900 sq ft
MaterialsEstimator, Home addition cost survey — by trade package · checked 2026-09-28
Every kind of addition
| Total | Per sq ft | |
|---|---|---|
| Bump-out, 10 × 3 ft | $15,108 | $504 |
| Bathroom, 8 × 10 ft | $54,356 | $679 |
| Sunroom, 14 × 16 ft | $57,810 | $258 |
| Room, 20 × 20 ft | $95,165 | $238 |
| Primary suite, 16 × 24 ft | $122,305 | $319 |
| In-law suite, 20 × 28 ft | $172,710 | $308 |
| Two-car garage, 24 × 24 ft | $67,044 | $116 |
| Second story, 30 × 30 ft | $236,802 | $263 |
The bathroom is the dearest square foot because its fit-out is priced like a bathroom remodel — about $320 a square foot of tile, waterproofing and fixtures — and it still pays the whole tie-in. The garage is the cheapest because it is a slab, a shell and a door. A second story trades the foundation for taking the roof off and reinforcing what is below.
MaterialsEstimator, Bathroom remodel budget survey — by scope tier and by trade · checked 2026-09-22
Foundation and finish
| 20 × 20 room | Foundation alone | |
|---|---|---|
| Slab on grade | $95,165 | $5,600 |
| Crawl space | $99,005 | $8,800 |
| Full basement | $111,485 | $19,200 |
The foundation usually follows the house, not the budget: a slab beside a house on a crawl space leaves the new floor a step down. Whatever the type, the footings go below the frost line. The finish level moves more: builder-grade to high-end takes the same room from $78,605 to $130,565, because it changes the windows and siding as well as the floors and trim.
International Code Council, International Residential Code, Section R403.1.4, Footing depth and frost protection · checked 2026-09-16
What the code and the lot decide
Zoning comes before cost: setbacks from the lot lines and the share of the lot buildings may cover decide how big the addition may be, and whether it goes out or up. Then the building code: a room anyone sleeps in needs an emergency escape window, and habitable rooms a ceiling of at least 7 ft. A bedroom may also need the septic system checked, and a large addition with its own heating a bigger electrical service.
International Code Council, IRC R310 — Emergency escape and rescue openings (2021) · checked 2026-09-25
International Code Council, International Residential Code, Section R305, Ceiling height · checked 2026-09-24
MaterialsEstimator, Ductless mini-split installed price survey · checked 2026-09-16
What goes wrong
Before asking for bids
- A survey of the lot with the setbacks marked.
- The kind of addition and its size.
- The foundation the house already sits on.
- How it will be heated and cooled.
- An egress window drawn into any bedroom.
- A contingency in the budget, not only in the builder's head.
01Budgeting from a price per square foot
A small addition costs far more per foot than a big one.
Instead — Price the pieces, as here.
02Leaving out plans and permit
They are about a tenth of the job.
Instead — Put them in from the start.
03Assuming the furnace will cope
A system sized to the house leaves the new room cold.
Instead — A load calculation, or its own mini-split.
04Spending the contingency on finishes
Opening the wall always finds something.
Instead — Keep 10% back until the framing is up.
- 01Check what the lot allowsSetbacks and lot coverage decide how far you can build out, and whether the answer is up instead.
- 02Pick the kind of additionRoom, bathroom, suite, in-law suite, sunroom, bump-out, garage or second story.
- 03Set the sizeWidth along the house and depth out from it; the area drives the foundation, shell and interior.
- 04Match the foundation to the houseSlab, crawl space or basement, so the floors line up.
- 05Choose the finishBuilder-grade, mid-range or high-end — it moves the shell and the interior.
- 06Add the fixed piecesTie-in, plumbing, heating, plans, permit and a contingency don't shrink with the size.
The same trade costs twice as much a foot on a small job: construction cost per square foot, calculated by trade.
Why a square foot lies- Tie-in
- Where the addition meets the house: the opening cut in the existing wall with a header over it, the new roof tied into the old, and the siding and shingles matched.
- Shell
- The framed structure and its envelope — walls, floor, roof, sheathing, windows, exterior door, siding and insulation — before any interior finish.
- Bump-out
- A small addition of 2–3 ft pushed out of one wall, usually cantilevered on extended floor joists so it needs no foundation.
- Cantilever
- A floor that projects beyond its support, carried by the joists behind it. Past about 3 ft it needs footings.
- Frost line
- The depth to which the ground freezes in winter. Footings go below it so the addition does not heave.
- Setback
- The minimum distance zoning requires between a building and the lot lines. It decides how far an addition may extend.
- Lot coverage
- The share of the lot buildings may cover under zoning — often what forces a second story instead of building out.
- Egress window
- An emergency escape opening every sleeping room needs, sized by clear opening and sill height.
- ADU
- Accessory dwelling unit: a second, self-contained home on the lot, attached or detached, with its own kitchen.
- Contingency
- Money held back for what opening the house reveals — rot, old wiring, a footing that is not where the plans said.
Common questions
- How much does a home addition cost?
- About $95,165 for a 20 × 20 ft room addition, mid-range, on a slab — $238 a square foot with plans, permit and a 10% contingency. The range is $68,157–$133,826. Builder-grade finishes bring it to $78,605; high-end to $130,565.
- How much does a home addition cost per square foot?
- About $197–$326 a square foot for a 400 sq ft room from builder-grade to high-end, and $238 mid-range. The square foot is not a rate: a 12 × 12 room is $277 a foot, a bathroom $679, a garage $116. What does not grow with the size — the tie-in, plumbing and heating — is spread over fewer feet on a small addition.
- How much does a 20 × 20 room addition cost?
- About $95,165 mid-range on a slab: $5,600 of foundation, $38,000 of shell, $28,000 of interior, $6,500 to tie it into the house, plus plans, permit and contingency. On a crawl space it is $99,005; over a full basement $111,485.
- How much does a 12 × 12 or 15 × 15 room addition cost?
- About $39,904 for 12 × 12 ft (144 sq ft) and $57,575 for 15 × 15 ft (225 sq ft), mid-range on a slab — $277 and $256 a square foot.
- How much does a bathroom addition cost?
- About $54,356 for an 8 × 10 ft bathroom: the shell is the cheap part, and the fit-out runs about $320 a square foot — the same as a same-layout bathroom remodel — plus about $3,500 to bring the drain, vent and water out to it.
- How much does a bedroom or primary suite addition cost?
- A 16 × 20 ft bedroom is about $77,981. A primary suite of 16 × 24 ft with an 80 sq ft bathroom and its own mini-split is about $122,305. Either way the bedroom needs an egress window.
- How much does a second story addition cost?
- About $236,802 for 900 sq ft mid-range — $263 a square foot. There is no foundation, but the roof comes off, the walls and footings below are checked by an engineer and reinforced ($28,000), and a staircase is added ($8,000). Most families move out for part of it.
- How much does a garage addition cost?
- About $67,044 for an attached 24 × 24 ft two-car garage with an insulated door and opener — $116 a square foot, the cheapest on this page because there is almost no interior finish — and $43,572 for a 14 × 24 ft single.
- How much does a laundry room addition cost?
- About $29,656 for an 8 × 10 ft laundry room: $26,156 for the room and about $3,500 to bring the water and a drain to it, plus a dryer vent and a 240 V circuit if the dryer is electric.
- How much does a sunroom or bump-out cost?
- A four-season sunroom of 14 × 16 ft is about $57,810, heated with a mini-split — the insulated glass is what costs. A 10 × 3 ft bump-out is about $15,108: cantilevered, no foundation, and after a bathroom the dearest square foot ($504), because the wall opening and roof cost the same as for a far bigger room.
- How much does an ADU or in-law suite cost?
- An attached in-law suite of 20 × 28 ft with a bedroom, a bathroom and a kitchenette is about $172,710. A detached ADU usually costs more a square foot: four walls instead of three, its own roof, and its own water, sewer and electrical connections.
- Is it cheaper to build up or out?
- Out, when the lot has room: 900 sq ft built on the ground beside the house is about $203,382 ($226 a square foot) against $236,802 ($263) for the same floor on top, and going up means the roof off and the family out. Up wins when setbacks or lot coverage leave no room, or when the yard is worth more than the difference.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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