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Home addition cost calculator

Work out what a home addition will cost from the square footage and a per-foot rate you set for the type of addition. A bump-out, a ground-floor room, and a second story are very different builds, so the calculator sums the build, the foundation, and the permit and design fees for the one you are planning.

§ 01 Your numbers

Change anything. The answer updates as you type.

The added floor area. A small bump-out might be 50 to 150; a room addition 200 to 500; a large suite more.
The all-in build rate for your type of addition. A simple ground-floor room is at the low end; a kitchen or bathroom addition, or a second story, runs much higher because of plumbing, structure, and finish.
A new foundation or footings for a ground addition, or reinforcing the house below for a second story. Zero if the per-foot rate already includes it.
Architectural drawings, structural engineering, the permit, and inspections. An addition is a major permitted build, and larger ones need stamped plans.
Estimated cost
$103,000

Typical range $82,400 – $149,350

  • Build (square feet × rate)$100,000
  • Foundation / structural work$0
  • Permits, design & engineering$3,000
  • Total$103,000
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$40,000 to $120,000 is a full room addition on a new foundation, possibly with plumbing. Get detailed bids and stamped plans.

What this assumes, and where it could be wrong

Every one of these is a place the number could be off. They are here because you should be able to check our working, not because we are hedging.

AN ADDITION COSTS MORE PER FOOT THAN THE HOUSE DID.
An addition is a small house grafted onto the existing one. It needs its own foundation, its own roof, its own walls, and it has to tie into the existing structure, plumbing, and electrical, which is fiddly, custom work. That is why the per-square-foot rate for an addition is usually higher than the cost of the original house per foot, and why a kitchen or bath addition costs far more than a plain room

The type of addition swings the rate more than the size. A bump-out that cantilevers off the existing floor is a cheaper build; a ground-floor room on a new foundation costs more; a second story is dearer because the house below must be reinforced and often vacated during the build; a kitchen or bathroom addition is the priciest per foot because of the plumbing and fixtures. Set the rate to your type.

Tying into the existing house is where surprises live. Matching the roofline, siding, and windows, moving or upgrading the electrical panel, and extending heating and cooling into the new space are real costs that a bare per-foot rate can miss. An addition that looks original costs more than one that looks bolted on.

A second story means living in a construction site or moving out. Building up usually means opening the roof and reinforcing the structure below, so the house may be exposed and partly unlivable for weeks. Factor the disruption, and possibly temporary housing, which is not in the build cost.

The defaults are ours and are a starting point. The size, the rate, and the extras are yours, and the estimate is only as good as a rate that matches the real type of addition and a design that has accounted for tying into the existing house.

Frequently asked questions

How much does a home addition cost?
It is priced by the square foot, and the rate depends heavily on the type. A simple ground-floor room is at the lower end per foot; a second story, or a kitchen or bathroom addition, is much higher because of the structure, plumbing, and finish. Multiply your size by a rate for your type, then add the foundation and the fees, which the calculator does from your numbers.
Why does an addition cost more per square foot than the house?
Because it is custom, small-scale construction that has to graft onto an existing building. It needs its own foundation, roof, and walls, plus the work of tying into the current structure, roofline, plumbing, and electrical, all done in a confined space around a house people are living in. That is more expensive per foot than building a whole house at once was.
Is it cheaper to add on or build up?
It depends. A ground-floor addition needs a new foundation and roof but is straightforward to build; a second-story addition skips the foundation but must reinforce the house below and often means the home is partly unlivable during the build. Building out is usually simpler and less disruptive; building up saves the yard. Neither is reliably cheaper without pricing your specific house.
Do I need permits and an architect for an addition?
Almost always a permit, and usually drawings, and for anything structural, an engineer. An addition changes the footprint and the structure, so it is inspected work with stamped plans in most areas. The design and permit fees are a real line, and skipping them risks the work not counting toward the home's value when you sell.

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