Room Addition Cost Calculator
A 12 × 16 foot single-storey addition runs about $51,438 in Atlanta — $32,986 to $69,890, or $268 a square foot. ⚠️ The rate falls fast with size: $391 a square foot at ten by ten against $194 at twenty by twenty, because the foundation, the roof tie-in and the drawings barely change. And 11% of the bill is not the new room — it is what the new room costs the old house.
Estimate the cost for your home
Estimated cost of the addition, finished
$52,937
typical · range $34,431 to $71,442 · 192 sq ft · $276 per sq ft · 243 trade hours
Labour priced at national wages. Enter a ZIP code to price it for your area. 74% of this figure comes from modelled material prices — each line shows its own uncertainty.
Priced from fema.gov Aug 2026bls.gov Aug 2026how every line is sourced
The one input that changes the answer for you.
The ordinary addition: new footings, new floor, new walls, new roof tied into the old one. It is the most expensive per square foot of the three and the one with fewest surprises, because everything is new and visible while it is being built. ⚠️ It also costs you garden, and on a tight plot the setback from the boundary is what decides whether it can be built at all — that is a question for the building department before it is a question for a builder.
Footings, short foundation walls and a framed floor over a vented or sealed void. It costs more than a slab and it earns some of that back: services stay reachable, the floor is warmer underfoot, and a future change is a hatch rather than a jackhammer.
Engineered wood or good vinyl plank, painted trim that matches the rest of the house, decent windows. What the calculator is calibrated on.
The three ways, side by side
| Out at ground level | 192 sq ft | $52,937 |
|---|---|---|
| Up | 192 sq ft | $64,155 |
| A small bump-out | 36 sq ft | $22,377 |
⚠️ The bump-out is priced at the size a bump-out can actually be — three feet deep — rather than at the size of the addition, because comparing them at equal area would be comparing against something that cannot be built.
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The figure above is already calculated. Each step just makes it more yours, and you can stop at any of them.
Why the small one costs more per square foot
| Addition | Area | Per sq ft | Cost |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | $402 | $40,217 |
| 12 × 12 ft | 144 sq ft | $326 | $46,915 |
| 12 × 16 ft | 192 sq ft | $276 | $52,937 |
| 16 × 20 ft | 320 sq ft | $217 | $69,470 |
| 20 × 20 ft | 400 sq ft | $200 | $79,971 |
Four times the floor area for about twice the money, and the reason is in the lines rather than in a discount. The foundation is priced by perimeter and perimeter grows with the square root of area. Tying the new roof into the old one costs the same whether the room behind it is 100 square feet or 400. So does opening the existing wall, and so does getting drawings a building department will accept. ⚠️ The practical consequence is worth acting on rather than admiring: if you are choosing between two sizes, the larger one almost never costs proportionally more — and the very small addition is the one where the fixed costs eat you.
What it does to the house you already have
| Tying the new roof into the existing one | $1,160 |
|---|---|
| Tying the new roof into the existing one — labour | $751 |
| Opening the existing wall and heading it off | $890 |
| Opening the existing wall and heading it off — labour | $1,056 |
| Protecting the house, skip hire and cleaning up | $1,650 |
| Protecting the house, skip hire and cleaning up — labour | $482 |
| Together, out of $52,937 | $5,989 · 11% |
None of this builds the new room. It is what the new room costs the old house, and the roof tie-in is the one to care about: cutting into an existing roof and framing the valley where the two meet is the highest-risk detail on the job. ⚠️ A valley done badly does not leak in year one — it leaks in year four, into the wall of the room you just built, underneath the finishes. Two more that are not on this table because they are not always needed and are not small: whether the panel has room for the new circuits, and whether the existing heating and cooling can actually serve the extra floor area. Extending ducts into a room does not add capacity to the system feeding them.
| Line item | Qty | Cost | Source |
|---|---|---|---|
| Drawings, structural calculations and the permit submission1 per project, except a bump-out | 1each | $1,400–$6,200 | estimated±63% |
| Excavation and site preparation for the new foundationfoundation perimeter × 0.09 (0.42 for a basement) | 3.6hour | $364–$422 | sourced |
| Footings, poured to frost depthfoundation perimeter in linear feet | 40linear ft | $960–$2,080 | estimated±37% |
| Footings, poured to frost depth — labourfoundation perimeter × 0.35 hours | 14hour | $745–$801 | sourced |
| Foundation walls, for a crawl space or basementfoundation perimeter, ×2.6 for a full basement | 40linear ft | $1,520–$3,840 | estimated±43% |
| Foundation walls, for a crawl space or basement — labourfoundation wall length × 0.42 hours | 16.8hour | $894–$961 | sourced |
| Framed floor over the crawl space or basementfloor area, over a crawl space or basement, or on a second storey | 192sq ft | $1,440–$2,880 | estimated±33% |
| Framed floor over the crawl space or basement — labourfloor area × 0.065 hours | 12.5hour | $706–$758 | sourced |
| Wall framing, sheathing and weather barrierexterior wall area in square feet | 360sq ft | $3,240–$6,480 | estimated±33% |
| Wall framing, sheathing and weather barrier — labourexterior wall area × 0.075 hours | 27hour | $1,527–$1,640 | sourced |
| Roof structure and deckingsloped roof area in square feet | 221sq ft | $1,768–$3,536 | estimated±33% |
| Roof structure and decking — laboursloped roof area × 0.07 hours | 15.5hour | $875–$940 | sourced |
| Tying the new roof into the existing one1 per addition, except a bump-out | 1each | $420–$1,900 | estimated±64% |
| Tying the new roof into the existing one — labour14 hours per addition, except a bump-out | 14hour | $724–$778 | sourced |
| Roof covering, underlayment and flashingsloped roof area in square feet | 221sq ft | $707–$1,658 | estimated±40% |
| Roof covering, underlayment and flashing — laboursloped roof area × 0.035 hours | 7.74hour | $401–$430 | sourced |
| Windows and exterior doorsnumber of window and door openings | 3each | $1,680–$5,550 | estimated±54% |
| Windows and exterior doors — labouropenings × 3 hours | 9hour | $509–$547 | sourced |
| Siding and exterior trimexterior wall area in square feet | 360sq ft | $1,980–$6,120 | estimated±51% |
| Siding and exterior trim — labourexterior wall area × 0.055 hours | 19.8hour | $1,120–$1,203 | sourced |
| Insulation and air sealingwall area + roof area | 581sq ft | $988–$2,440 | estimated±42% |
| Insulation and air sealing — labour(wall area + roof area) × 0.018 hours | 10.5hour | $480–$515 | sourced |
| Plasterboard, taped and finishedinterior wall and ceiling area | 696sq ft | $1,044–$2,088 | estimated±33% |
| Plasterboard, taped and finished — labourinterior area × 0.032 hours × lead factor | 22.3hour | $1,225–$1,316 | sourced |
| Flooring, trim and decorationfloor area × finish factor | 192sq ft | $1,536–$4,032 | estimated±45% |
| Flooring, trim and decoration — labourfloor area × finish factor × 0.09 hours | 17.3hour | $797–$856 | sourced |
| Circuits, outlets, lighting and smoke alarmsfloor area in square feet | 192sq ft | $499–$1,152 | estimated±40% |
| Circuits, outlets, lighting and smoke alarms — labourfloor area × 0.055 hours | 10.6hour | $623–$669 | sourced |
| Extending the existing heating and cooling into the addition1 when the existing system is extended | 1each | $900–$3,200 | estimated±56% |
| Extending the existing heating and cooling into the addition — labour14 hours when the existing system is extended | 14hour | $798–$858 | sourced |
| Opening the existing wall and heading it off1 when the existing wall is opened up | 1each | $380–$1,400 | estimated±57% |
| Opening the existing wall and heading it off — labour18 hours × lead factor, when the wall is opened | 18hour | $1,018–$1,093 | sourced |
| Protecting the house, skip hire and cleaning up1 per project | 1each | $700–$2,600 | estimated±58% |
| Protecting the house, skip hire and cleaning up — labourfloor area × 0.055 hours | 10.6hour | $465–$499 | sourced |
| Total | $34,431–$71,442 | ±35% |
Where these numbers come from
- Why the small addition costs so much per square foot
- The three questions Google shows alongside this search are all sizes — ten by ten, twelve by twelve, twenty by twenty — and the useful answer is not three prices, it is the shape of the curve. $391 a square foot at 10 × 10, and $194 at 20 × 20 — four times the area for about 2 times the money. The foundation is priced by perimeter, and perimeter grows with the square root of area. Cutting into the existing roof and framing the valley where the two meet costs the same either way. So does opening the existing wall, and so does getting drawings a building department will accept. ⚠️ The practical consequence is worth acting on: if you are choosing between two sizes, the larger one almost never costs proportionally more, and the very small addition is the one where the fixed costs eat you alive.
- What the addition does to the house you already have
- $5,679 of this job — 11% — builds none of the new room. It is the roof tie-in, opening the existing wall and heading it off, and protecting the part of the house you are still living in. The roof tie-in is the one to care about: cutting into an existing roof and framing the valley where old meets new is the highest-risk detail on the job, and a valley done badly does not leak in year one — it leaks in year four, into the wall of the room you just built, underneath the finishes. ⚠️ Two more that are not always needed and are never small. Whether the panel has room for the new circuits, priced on our electrical panel upgrade cost calculator. And whether the existing heating and cooling can serve the extra floor area at all: extending ducts into a room does not add capacity to the system feeding them, and the result is a new room that never gets comfortable. Our HVAC replacement cost calculator prices the alternative.
- Out, up, or neither
- Three ways to gain the space, and they are not variations on a theme. At ground level on new foundations: $51,438. As a second storey: $62,609 — dearer, which surprises people, because no foundation is the only cheap part. The existing roof comes off, the house sits under a temporary cover while it does, the walls and footings below have to be shown to carry the new load, and a staircase eats a room downstairs to reach the rooms upstairs. ⚠️ And the third: a 36 square foot cantilevered bump-out at $21,798, carried on the joists that are already there. It is genuinely the cheapest way to gain usable space and it is limited by physics rather than budget — the overhang reaches about a third of the span behind it. Where it fits, nothing else competes. The foundation choice matters too: $47,278 on a slab, $51,438 on a crawl space and $58,250 over a full basement — and that last one gives the square footage twice only if the basement is going to be finished, which our basement finishing cost calculator prices.
- Insulation is not the place to save, and where the wages come from
- New conditioned space has to meet the energy code in force where you live, which is a real specification with a real inspection — DOE Building Energy Codes Program, State Energy Code Methodology (retrieved 2026-08-24) tracks which code each state has adopted, and ENERGY STAR, Insulation Tax Credit (retrieved 2026-08-24) covers the product side. ⚠️ It is the cheapest line on this page to get right and the most expensive to get wrong, because it is buried behind plasterboard for the life of the building. Notice, too, how little the finish level moves the total: $50,423 basic against $53,431 high — it multiplies the flooring, the trim and the decoration and nothing structural. If the budget is tight, this is not where it gets fixed. In a house built before 1978, cutting into painted surfaces brings the work inside EPA, Lead Renovation, Repair and Painting Program (retrieved 2026-08-24). Wages from BLS OEWS, May 2025, retrieved 2026-08-20, benefit-loaded from BLS ECEC. And if the cheapest way to gain a room is not to build one, our garage conversion cost calculator is the other side of that decision.
- Material prices
- Modelled estimates, not sourced prices. No public, machine-readable source of US residential material prices exists, and we would rather say so than pretend otherwise. Each line carries its own uncertainty band. How we estimate them.
Contractor overhead and profit applied at 33–43% over direct cost. Data last reviewed 2026-08-24.
What this figure does not include
- The permit fee, and the inspections
- An addition is one of the most inspected jobs a homeowner ever commissions — footing, framing, insulation, electrical, plumbing and final. The fee is often a percentage of the declared job value and varies by an order of magnitude between jurisdictions, so there is no national figure worth publishing.
- Whether you are allowed to build it at all
- ⚠️ Setbacks from the boundary, lot coverage limits, height limits and, where there is one, a homeowners association. These decide the shape of an addition before any budget does, and the answer is a phone call to the building department rather than a number on a page.
- A service upgrade if the panel is full
- The electrical line assumes the existing panel has room and capacity for the new circuits. Where it does not, that is a service upgrade priced on our electrical panel page — and it is worth checking before the drawings, because it is a four-figure item that has nothing to do with the size of the room.
- A bigger heating and cooling system for the whole house
- Extending ducts into a new room does not add capacity to the system feeding them. If the existing equipment is already at its limit, the addition turns into a whole-house HVAC decision, priced on our HVAC and heat pump pages.
- Moving the drains, or the septic system
- An addition that contains a bathroom or a kitchen is a different project — our bathroom addition page prices the plumbing side of it. On a septic system, adding a bedroom can require the tank and field to be re-sized, which is a permit question and a real cost.
- Re-roofing or re-siding the rest of the house to match
- Priced on our roof replacement and siding pages. It is excluded here and it belongs in the decision: if either is near the end of its life, doing it with the addition is much cheaper than doing it separately two years later.
- Living somewhere else while it happens
- A ground-floor addition is disruptive; a second storey usually means moving out for weeks. That is a real cost of the decision and it appears on no estimate.
Questions people actually ask
How much would a 10x10 room addition cost?
About $39,088 on this model in Atlanta, single storey on a crawl space — which is $391 a square foot, the most expensive rate on the whole page. ⚠️ That is not a small-job surcharge, it is arithmetic, and it is the most useful thing to understand before commissioning one. Several of the big lines do not shrink with the room: tying the new roof into the existing one costs the same at 100 square feet as at 400, so does opening the existing wall, and so do drawings a building department will accept. On a ten by ten those fixed items are a much larger share of a much smaller total. If a slightly bigger room would be useful, it is nearly always better value than the one you are pricing.
How much does a 12x12 room addition cost?
$45,586 on this model in Atlanta — $28,850 to $62,322 — for a single-storey addition on a crawl space with a standard finish, three openings, the existing wall opened up and the heating extended. That is 211 trade hours across seven trades. Of the total, $5,679 is not the new room at all: it is what the new room costs the old house — the roof tie-in, opening the wall, and protecting the part you keep living in. ⚠️ Two things this figure assumes and a quote might not: that your electrical panel has room for the new circuits, and that the existing heating and cooling can actually serve the extra floor area. Extending ducts into a room does not add capacity to the system feeding them.
How much would a 20x20 room addition cost?
$77,683 on this model in Atlanta, and the interesting number is the other one: $194 a square foot against $391 for a ten by ten. Four times the floor area for about 2 times the money. The foundation is priced by perimeter and perimeter grows with the square root of area; the roof tie-in and the wall opening do not grow at all. ⚠️ At this size the questions that matter stop being about the room and start being about the house: a 400 square foot addition is a real load on a heating system, a real draw on a panel, and — if it contains a bedroom on a septic system — sometimes a reason the tank and field have to be re-sized.
What is the cheapest way to add a room onto a house?
On our own models, in order: convert something you already have, then bump out, then build. A garage conversion of similar floor area is priced on our garage conversion page and comes in well under an addition, because a garage already has a foundation, walls and a roof. A cantilevered bump-out — 36 square feet at $21,798 here — is the cheapest thing on this page in absolute terms, and it is limited: the overhang can only reach about a third of the joist span behind it, so it buys a breakfast nook or a bigger bathroom, not a bedroom. ⚠️ And the option people expect to be cheap is the dearest: a second storey is $62,609 against $51,438 at ground level, because taking the existing roof off, proving the walls below carry the load and fitting a staircase costs far more than a foundation.
Taken from Google’s “people also ask” results for this search on 2026-08-24, not invented.
Other cost calculators
The first of these are the jobs closest to this one — the ones that turn up in the same quote, or instead of it.
- Garage conversion cost
Prices the floor a garage slab really needs — it is sloped and set low on purpose — and everything a change of use brings that a refit does not.
- Bathroom addition cost
Prices the drain, the vent and the joist work no quote itemises, and the pump a bathroom below the sewer line cannot do without.
- Walk-in shower cost
Prices a walk-in shower from its own dimensions, checks them against the published ADA sizes, and shows what a curbless entry really costs.
- The full room addition cost dataset
Every unit price, every quantity rule and the reasoning behind each estimate, as a plain table.
- How we calculate
The formula, the sources, and an honest account of which parts of the model are weak.