Home Addition Cost Calculator
Home Addition costs $63,475 to $95,975 for a typical 400 sq ft job in 2026, against a national benchmark of $77,400. Labor runs 460 crew hours at $38.25 an hour. Enter a ZIP code to apply that area's measured wages for this work and its regional price level.
What makes up the price
Cost by size
National averages, before any local adjustment. Computed from the same model as the calculator above, so the two always agree.
| Addition size | Sq ft of addition | Typical | Range |
|---|---|---|---|
| 225 sq ft | 225 | $48,125 | $39,475 to $59,675 |
| 325 sq ft | 325 | $64,850 | $53,175 to $80,425 |
| 400 sq ft | 400 | $77,400 | $63,475 to $95,975 |
| 550 sq ft | 550 | $102,475 | $84,025 to $127,075 |
| 675 sq ft | 675 | $123,375 | $101,175 to $153,000 |
Before anything else: three comparisons
An addition quote arrives on its own. These are the numbers it should be sitting next to.
Against finishing a basement. Finishing 800 sq ft of existing basement is around $48,500 — about $61 a square foot against $194 for new space on a slab. If the space you want could be down there, that is a third of the price, because the foundation and the roof are already paid for.
Against the appraisal. An addition typically recoups 50 to 65% of its cost at resale, which is among the weakest figures in remodeling. A $77,400 addition adds something like $40,000 to $50,000 of appraised value. That can be a perfectly good decision — you get to live in it for fifteen years — but it should be a decision rather than a discovery.
Against moving. At $77,400 plus months of disruption, compare the transaction cost of selling and buying: agent commission, closing costs, moving, and the difference in price for a house that already has the room. In a lot of markets those numbers are closer than people expect, and in some the addition wins clearly. Do the arithmetic before the design.
None of this is an argument against building. It is an argument for knowing which of the three you are choosing.
The foundation is the biggest decision
| 400 sq ft addition | Per sq ft | |
|---|---|---|
| Slab on grade | $77,400 | $194 |
| Crawl space | $82,275 | $206 |
| Full basement | $94,150 | $235 |
| Build up instead | $125,200 at 600 sq ft | $209 |
Slab on grade is cheapest and has one real drawback: the finished floor sits near grade, which usually means a step down from the existing house. Fine for a sunroom or a garage, awkward for a bedroom you walk to at night.
A crawl space costs about $5,000 more on a 400 sq ft addition and buys two things: the new floor meets the existing one at the same level, and the plumbing has somewhere to run. For a bedroom or a bathroom it is usually the right answer.
A full basement is $17,000 more and is the only foundation that gives square footage back. If you need the space more than you need the money, it is the cheapest basement you will ever build — the excavator is already there.
Building up avoids a foundation entirely and buys structural work instead: new load finding its way down through the house to footings that were sized for one story, plus the existing roof coming off and the house being made weathertight every night in between. It also preserves the yard, which on a small lot is sometimes the whole argument.
Small additions are expensive per foot
A 150 sq ft addition is about $237 a square foot and an 800 sq ft one about $180. The reason is that the permit, the structural engineering, the roof tie-in and the mobilization do not shrink with the room.
The practical consequence: if you are considering 200 square feet, price 350 as well. The second number is often much less than proportionally more, and the answer sometimes changes what gets built.
The roof tie-in is the cost people forget
A new roof has to join an existing one. How hard that is depends on what is already there:
- A gable off a flat wall is the easy case and the baseline here.
- Tying into a hip roof creates a valley, which is skilled work and the single most likely place for the finished house to leak. About 12% more.
- Matching an older house — brick coursing, trim profiles, a window style, an unusual roof pitch — is 25% more, and on a genuinely old house some of what has to be matched is not manufactured any more and has to be made.
Related, and also forgotten: the existing house has to keep working. Rooms lose their windows. A wall that was an exterior wall becomes an interior one, which means insulation and siding coming off it and a header going in where the opening is cut. And the heating and cooling has to reach the new space — if the existing system has no spare capacity, the addition has quietly become an HVAC project too.
What the number here does not include
Stated plainly, because an addition has more adjacent costs than anything else on this site:
- Impact and utility connection fees, which some jurisdictions charge on new conditioned square footage and which can run into thousands.
- Architectural design, if the project needs more than a builder's permit set. $3,000 to $15,000.
- Landscaping and hardscape put back after the excavator has been across it.
- Furnishing the room.
- A larger HVAC system, if the existing one cannot carry the load.
- Setback, easement and zoning limits, which can simply make a particular footprint impossible — this is the first thing to check and it is free. Call the building department with your address before you draw anything.
The timeline
Four to eight months from first conversation to finished room, of which only two to four are building:
- Design and engineering: three to eight weeks.
- Permit review: two to twelve weeks, and this is entirely out of the contractor's control.
- Construction: eight to sixteen weeks for a single-story addition, longer for a second story.
- Weather, which on a project with an open roof is not a detail.
A second story is worse than the elapsed time suggests, because the house is open at the top. Most second-story projects mean moving out for part of it, and that cost belongs in the budget.
Questions worth asking
- What are the setbacks on this lot, and does the footprint fit? Ask the building department, free, first.
- Is the quote fixed price or cost-plus? On a project this size the difference matters more than the number.
- What is excluded? Ask for the exclusions as a written list.
- Who carries the engineering, and is it in the price?
- Does the existing HVAC have capacity, and who has calculated that?
- What is the draw schedule, and is it tied to completed stages rather than dates?
- Is there a written change-order process? On a four-month project there will be changes, and the time to agree how they get priced is before the first one.
- Can I see two additions you finished more than three years ago? Three years is long enough for the roof tie-in to have been tested.
Common questions
How much does a home addition cost?
Around $77,400 for a 400 sq ft room addition on a slab, with most jobs that size falling between $63,475 and $95,575 — about $194 per square foot. A crawl space foundation adds roughly $5,000 and a full basement about $17,000. Adding a bathroom takes the same addition to about $96,750, and an 800 sq ft addition comes to around $144,300.
Is a home addition worth the money?
As an investment, no: additions recoup roughly 50 to 65% of their cost at resale, which is among the weakest figures in remodeling, so a $77,400 addition adds something like $40,000 to $50,000 of appraised value. As a purchase it can be entirely reasonable — you get to live in the space for fifteen years. The point is to know which one you are doing. And compare it against two alternatives first: finishing a basement, at about a third the cost per square foot, and moving.
Is it cheaper to build an addition or finish the basement?
Finishing a basement is roughly a third of the cost per square foot — about $61 against $194 for new space on a slab — because the foundation and the roof are already paid for. If the space you want could plausibly be in the basement, that comparison should happen before any addition is designed. The catch is that below-grade space is valued lower by appraisers and does not suit every use, so it is a genuine trade rather than a free win.
Which foundation should an addition have?
A crawl space, usually, for habitable space. A slab is cheapest but puts the finished floor near grade, which generally means a step down from the existing house — fine for a sunroom, awkward for a bedroom. A crawl space costs about $5,000 more on a 400 sq ft addition and lets the floors meet while giving the plumbing somewhere to run. A full basement is about $17,000 more and is the only option that returns usable square footage, which makes it the cheapest basement you will ever build if you want the space.
Is a second-story addition cheaper than building out?
Per square foot it is comparable — around $209 against $194 on a slab — because it trades the foundation for structural work: new load finding a path down through the existing house to footings sized for one story, the existing roof coming off, and the house being made weathertight every night in between. What it really buys is the yard, which on a small or constrained lot is often the whole argument. What it costs beyond money is that the house is open at the top, so most second-story projects mean moving out for part of the work.
Why do small additions cost so much per square foot?
Because the permit, the structural engineering, the roof tie-in and getting the crew mobilized do not shrink with the room. A 150 sq ft addition is about $237 a square foot and an 800 sq ft one about $180. The practical move: if you are considering 200 square feet, price 350 as well. The bigger number is often much less than proportionally more, and that sometimes changes what gets built.
What is a roof tie-in and why does it cost extra?
It is where the new roof joins the existing one, and it is the cost people most reliably forget. A gable off a flat wall is straightforward. Tying into a hip roof creates a valley, which is skilled work and the most likely place on the finished house to leak — about 12% more. Matching an older house, with its brick coursing, trim profiles and unusual roof pitch, is about 25% more, and on a genuinely old house some of what has to be matched is no longer manufactured.
What does an addition quote usually leave out?
More than on any other job. Impact and utility connection fees that some jurisdictions charge on new conditioned square footage. Architectural design beyond a builder's permit set, at $3,000 to $15,000. Landscaping put back after the excavator has crossed it. Furnishing the room. And a larger HVAC system if the existing one cannot carry the extra load, which is a real possibility nobody mentions until the room is framed. Ask for the exclusions as a written list.
Do I need an architect for a home addition?
Not always. A straightforward single-story addition can often be done from a builder's permit set with a structural engineer sizing the framing, which is in the figures here. An architect becomes worth the $3,000 to $15,000 when the addition has to look as though it was always there, when the roof geometry is difficult, when the layout needs real thought, or when a design review board is involved. On a second story it is usually not optional.
How long does a home addition take?
Four to eight months end to end, of which only two to four are building. Design and engineering is three to eight weeks, permit review two to twelve and entirely outside the contractor's control, construction eight to sixteen weeks for a single story. Weather matters on a project with an open roof in a way it does not on an interior remodel. Build the permit wait into your expectations rather than treating it as a delay.
Can I build an addition anywhere on my lot?
No, and this is the first thing to check because it is free and it can end the project. Zoning sets setbacks from each property line, a maximum lot coverage, and sometimes a height limit; easements for utilities or drainage can cross the yard and cannot be built over. Call the building department with your address before anything is drawn. People spend money on designs for footprints that were never permissible.
Should the addition be fixed price or cost-plus?
On a project this size the answer matters more than the number. Fixed price puts the risk of the unknown on the contractor, who prices it in — so it costs more on paper and you know what you are spending. Cost-plus is transparent and open-ended, which works with a contractor you trust and a clear scope, and badly otherwise. Whichever you choose, insist on a written change-order process and a draw schedule tied to completed stages rather than calendar dates: on a four-month project there will be changes, and the time to agree how they get priced is before the first one.
Where these numbers come from
Nothing here is copied from a contractor directory. Labor is priced from what the trades are actually paid, materials from published producer prices, and both are adjusted to your area using federal cost-of-living data.
- Residential remodelers, United States: $38.25/hour, August 2026 (BLS CES CES2023611803)
- Framing, sheathing, roof, windows and siding: $38.00 per sq ft of addition (price recorded October 2026, not index-adjusted)
- Drywall, flooring, trim, wiring and HVAC extension: $28.00 per sq ft of addition (price recorded October 2026, not index-adjusted)
- Slab-on-grade foundation: $7.00 per sq ft of addition (price recorded October 2026, not index-adjusted)