The short answer
An addition is new construction attached to an old house, and it is priced like both. You pay new-construction costs for the foundation, framing, roof, and finishes, and remodeling costs for opening the existing wall, matching the siding and roofline, and extending the electrical, plumbing, and heating systems. That combination is why additions cost more per square foot than building a new house.
Three things set the total: the type of addition, its size, and whether it contains plumbing. A bedroom is mostly framing and drywall. Add a bathroom and the same square footage costs tens of thousands more. On top of construction come design fees, permits, and a contingency, which together commonly add 20% to 30%. The calculator above shows each layer.
How the calculator works
- Structure and finishes = floor area × the cost per square foot for the addition type × a finish factor (1.0 standard, 1.15 upgraded, 1.4 high-end) × the local cost level.
- Foundation upgrade. The base price assumes a concrete slab. A crawlspace or full basement adds a cost per square foot for ground-level additions.
- Bathrooms, kitchen, and heating and cooling are added as flat amounts, in addition to the floor area they occupy.
- Design and engineering is 5% to 12% of construction when selected. Permits and fees are a flat allowance.
- Contingency is your chosen percentage of everything above.
The per-square-foot construction figures already include the general contractor's overhead and profit, because additions are almost always built by one.
Cost assumptions
Construction cost per square foot at a standard finish level, national baseline. These are planning ranges compiled from published 2026 addition cost guides and checked against national benchmark projects, not quotes. Published figures for additions vary more than for any other project on this site.
| Addition type | Low | Typical | High |
|---|---|---|---|
| Room addition, ground level | $150 | $225 | $350 |
| Bump-out | $200 | $350 | $550 |
| Second-story addition | $200 | $325 | $500 |
| Four-season sunroom | $180 | $280 | $420 |
| Attached garage | $60 | $95 | $150 |
| Item | Unit | Low | Typical | High |
|---|---|---|---|---|
| Crawlspace foundation | Per sq ft | $10 | $15 | $25 |
| Full basement foundation | Per sq ft | $40 | $60 | $90 |
| Full bathroom | Each | $12,000 | $20,000 | $32,000 |
| Half bathroom | Each | $6,000 | $10,000 | $16,000 |
| Wet bar or kitchenette | Flat | $8,000 | $15,000 | $25,000 |
| Full kitchen | Flat | $20,000 | $35,000 | $60,000 |
| Extend existing heating and cooling | Flat | $2,000 | $4,000 | $7,000 |
| Ductless mini-split | Flat | $3,500 | $5,500 | $8,500 |
| Separate heating and cooling system | Flat | $8,000 | $12,000 | $18,000 |
| Design and engineering | Share of construction | 5% | 8% | 12% |
| Permits and fees | Flat | $1,000 | $2,500 | $6,000 |
Seven additions, priced
National baseline, design fees included, 10% contingency, existing heating and cooling extended unless noted.
| Project | Low | Typical | High |
|---|---|---|---|
| Bump-out, 40 sq ft | $12,700 | $24,100 | $42,300 |
| Attached two-car garage, 440 sq ft, unheated | $31,600 | $52,400 | $87,900 |
| Four-season sunroom, 200 sq ft, mini-split | $46,700 | $75,800 | $120,600 |
| Room addition, 300 sq ft | $55,400 | $87,700 | $144,600 |
| Primary suite, 400 sq ft, full bath, crawlspace, upgraded finishes | $101,600 | $161,300 | $265,300 |
| In-law suite, 500 sq ft, full bath, kitchenette, mini-split | $114,900 | $184,500 | $302,900 |
| Second story, 600 sq ft, full bath | $155,900 | $262,900 | $424,200 |
As a check, the 2025 Cost vs. Value Report prices a midrange primary suite addition at $170,517 nationally, close to the typical figure above.
Types of addition
- Room addition. A new room on its own foundation with its own roof, opened to the house through an existing exterior wall. The most common type and the baseline for comparison.
- Bump-out. A small extension of one room, often two to four feet deep, to fit a larger kitchen, a bathroom, or a closet. Some can be cantilevered off the existing floor framing without a foundation. The total is modest, but the cost per square foot is the highest of any type.
- Second story. New living space above all or part of the house. The roof comes off, the walls and foundation below may need reinforcing, and a staircase takes space from the first floor.
- Sunroom. A four-season sunroom is insulated and conditioned like any other room, with far more glass. A three-season room without heat costs much less and is not counted as living area.
- Attached garage. The least expensive space to build because it is unconditioned and unfinished. Fire-rated drywall and a rated door are required where it meets the house.
- In-law suite. A room addition with a bathroom and a kitchenette or kitchen. Once it has a full kitchen and its own entrance, many jurisdictions treat it as an accessory dwelling unit with separate zoning rules, utility requirements, and fees.
Building up or building out
Building out is usually cheaper per square foot and far less disruptive. Most of the work happens outside the existing walls until the final connection. Its limits are zoning and land: setbacks from the property lines, maximum lot coverage, easements, septic fields, and trees.
Building up uses no yard and needs no new foundation, but it puts new weight on a structure that was not designed for it. An engineer has to confirm that the foundation and first-floor walls can carry the load, and reinforcing them can be a large hidden cost. The house is open to the weather during framing, the first-floor ceilings are usually damaged, and most families move out for part of the job. Height limits apply.
Before either, look at the space you already have. Finishing a basement or attic, or converting a garage, typically costs a fraction of an addition because the shell exists.
What drives the price
- Plumbing. Every bathroom and kitchen adds supply, drain, and vent lines, fixtures, waterproofing, and tile. Distance from the existing plumbing matters.
- Foundation and site. Sloped lots, poor soil, high water tables, and limited access for excavation equipment all raise the cost of getting out of the ground.
- Tying in. Matching the existing roofline, siding, windows, and floor height takes more labor than building a box, and discontinued siding or brick may need custom sourcing.
- Existing systems. If the electrical panel, furnace, or septic system has no spare capacity, the addition pays to upgrade it. A septic system is sized by bedroom count, so adding a bedroom can require a larger system.
- Finish level. Flooring, cabinetry, tile, windows, and trim can move the price by 40% or more for the same floor plan.
- Size. Larger additions cost more in total and less per square foot.
Costs people forget
- Survey and soil report, often required for the permit.
- Utility upgrades: a larger electrical service, a new panel, or a bigger water heater.
- Remodeling the adjoining room, which nearly always follows once a wall is opened.
- Landscaping, driveway, and fence repair after heavy equipment leaves.
- Temporary housing or storage, especially for a second story.
- Higher property tax and insurance every year afterward.
- Financing costs if the project is paid for with a home equity loan or a construction loan.
Zoning, permits, and timeline
Check zoning before paying for full plans. Your building or planning department can tell you the setbacks, height limit, and lot coverage that apply to your lot, and whether you are in a historic district, a flood zone, or a homeowners association with its own design review. If the addition does not fit, the options are to redesign it or apply for a variance, which takes months and is not guaranteed.
A typical schedule is one to three months of design, one to three months of plan review, and three to six months of construction for a ground-level addition. A second story commonly takes six to nine months to build. Our renovation planning guide covers sequencing, and the guide to finding the right contractors covers selecting a builder.
Is an addition worth it?
On resale numbers alone, rarely. In Zonda's 2025 Cost vs. Value Report, a midrange primary suite addition costing $170,517 added $55,097 to resale value, about 32%, and an upscale one costing $351,613 added about 18%. A midrange bathroom addition costing $60,645 recovered about 53%. The renovation ROI calculator compares these with other projects.
The better comparison is with moving. An addition makes sense when:
- You plan to stay long enough to use the space for many years.
- You like the location, school district, and lot, and comparable larger homes nearby cost much more.
- The finished house will not be far larger or more expensive than its neighbors.
- Selling and buying would cost more: run the home sale proceeds calculator to see what a sale would leave you.
After it is built
- Update your insurance. An addition raises the cost to rebuild your home, and your dwelling limit needs to rise with it. The home insurance calculator recalculates your replacement cost and checks the 80% rule.
- Keep the records. An addition is a capital improvement, and at these amounts the paperwork is valuable. Its full cost is added to your home's basis, which can reduce the taxable gain when you sell. The capital gains on home sale calculator shows the effect. Keep the plans, contract, every paid invoice, the permit and final inspection, and photos of the framing and mechanical rough-in before the walls were closed.
- Expect a reassessment. The permit notifies the assessor, and your property tax will reflect the added value.
What the estimate leaves out
The calculator does not price structural reinforcement of the existing house, foundation repair, demolition of existing structures, major site work, septic or well upgrades, electrical service upgrades, remodeling of adjoining rooms, landscaping, temporary housing, impact fees, financing costs, or detached accessory dwelling units. Treat the result as a range to test builder proposals against.
Cost assumptions last reviewed November 2026. See our calculator methodology.