Home Addition Cost Calculator

Estimate a home addition by type and size, with the foundation, the kitchen or bath inside it, the HVAC extension and the soft costs that a per square foot number leaves out.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To budget an addition, price the shell per square foot, add the foundation, any bath or kitchen and the HVAC, then add soft costs and contingency. A 400 sq ft single story addition on a crawlspace runs about $112,000 to $241,700, or $280 to $604 per sq ft.

Your measurements

Plan view from above: area is length times width.

Added floor areasq ftFinished area of the new space. A 20 x 20 ft room is 400 sq ft, which is the most common single story addition.Addition typeA garage conversion is the cheapest square footage in residential work because the foundation, walls and roof already exist.FoundationOnly for additions that touch the ground. A basement adds the most per square foot but returns finished space, unlike a slab or crawlspace.Full baths in the additionEach full bath typically adds $15,000 to $30,000 installed. A half bath is roughly half that. Distance to the existing stack drives it.Kitchen in the addition%Percent of a full kitchen. A kitchenette is about 40, a full kitchen 100. A full kitchen typically adds $25,000 to $75,000.HVAC approach%0 extends existing ducts where the system has capacity, 50 adds a mini split, 100 is a new system or zone. Typically $3,000 to $18,000 depending on the answer.Design, permits and engineering%Architect or designer, structural engineer, survey, permit fees and plan review, as a percent of construction. 8 to 15 percent is normal on a residential addition.Contingency%Additions open up the existing house, so surprises are the rule. 15 percent is a floor on an older home and 20 is safer.

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Results

Total range: $112,000 to $241,700; 400 sq ft 1 story on a crawl foundation, with 12% soft costs and 15% contingency
All in per square foot: $280 per sq ft to $604 per sq ft; The shell alone is $180 to $400. New foundation, walls, roof and systems on ground the house does not currently sit on
Construction: $86,960 to $187,680; Shell $72,000 to $160,000, foundation $8,960 to $15,680, HVAC
Soft costs: $10,435 to $22,522; Design, structural engineering, survey, permit and plan review. Paid before a shovel moves, and not refundable if the project stops
Contingency: $14,609 to $31,530; Held back for what opening the existing wall reveals: old wiring, undersized framing, rot at the band joist, a foundation that is not where the plan says

Home Addition Cost formula

Construction = Area x Shell rate + Area x Foundation rate + Baths x $15,000 to $30,000 + Kitchen share + HVAC
Soft costs = Construction x Design and permit percent
Contingency = (Construction + Soft costs) x Contingency percent
Total = Construction + Soft costs + Contingency

Worked example

A 250 sq ft second story addition over an existing garage: no new foundation, one full bath, no kitchen, a mini split for conditioning, 12 percent soft costs and 20 percent contingency.

The shell at $200 to $450 per square foot is $50,000 to $112,500. The bath adds $15,000 to $30,000 and the mini split $6,000 to $12,000, so construction is $71,000 to $154,500. Soft costs add $8,520 to $18,540 and the contingency $15,904 to $34,608. The total lands near $95,400 to $207,600, which is $382 to $831 per square foot: small additions always cost more per foot because the fixed work does not shrink.

Quick reference

Typical 2026 US shell cost per square foot before foundation, baths, soft costs and contingency
Addition type	Per sq ft	What drives it
Garage conversion	$90 to $200	Shell exists; floor, insulation, windows and systems
Sunroom or four season room	$150 to $350	Glass and its structure, plus how it is conditioned
Single story addition	$180 to $400	New foundation, walls, roof and full systems
Bump-out, cantilevered	$200 to $400	No foundation, but high fixed cost over few square feet
Second story addition	$200 to $450	Existing foundation and walls must carry the new load

Why per square foot is a starting point, not an answer

Every addition quote is eventually expressed per square foot, but that number is an output rather than an input. The same 400 square feet costs wildly different amounts depending on whether it needs a foundation, whether it contains plumbing, and whether the existing systems can carry it. A garage conversion at $90 a foot and a second story with a bathroom at $450 are both honest numbers for real jobs.

The other reason the per foot figure misleads is scale. Mobilizing a crew, tying into the existing roof, running the permit and setting a foundation cost close to the same on 200 square feet as on 600. A small addition therefore always shows a higher figure per foot, which is exactly why bump-outs look expensive on paper and still make sense when all you need is four more feet of kitchen.

The three costs people leave out

Soft costs come first and hurt most, because they are spent before anything is built. A designer or architect, a structural engineer for anything that changes a load path, a boundary survey, plan review and permit fees typically add 8 to 15 percent of construction, and none of it is recoverable if the project stalls.

Systems come second. An addition is new load on an old house: the furnace may not have the capacity, the panel may not have the spaces, and the water heater may not keep up with another bathroom. Third is the contingency, which on a house over 40 years old is not optional. Opening an exterior wall is when knob and tube wiring, an undersized header, a rotted band joist or a foundation that does not match the drawings turns up. Fifteen percent is a floor and twenty is comfortable.

What this calculator leaves out

Site work: excavation, tree removal, utility relocation, and any retaining or drainage the new footprint forces. It does not price finishes above a normal builder standard, custom windows, a fireplace, or matching an unusual existing siding or roof. Landscaping and hardscape repair after the crew leaves, temporary housing if the kitchen is out of service, and property tax reassessment on the added square footage are all outside it, as are the setback, easement and lot coverage limits that decide whether the addition is allowed at all.

Code limits that apply

IRC R304.1
A habitable room is at least 70 sq ft, with no horizontal dimension under 7 ft
If you miss it: A narrow bump-out reads as a closet on the plan review and does not count as a bedroom
IRC R305.1
7 ft ceiling height in habitable rooms, measured to the lowest projection
If you miss it: A second story built inside an existing roofline runs short on height at the eaves
IRC R310.1
Emergency escape opening in every sleeping room, 5.7 sq ft net clear
If you miss it: A new bedroom whose only window is a small fixed unit fails and the opening has to be enlarged
IRC Table R301.5
Live loads the new floor must carry: 40 psf living areas, 30 psf sleeping rooms
If you miss it: A second story framed on the existing walls without an engineer's check overloads the foundation below
IRC R105.2
Additions are never permit exempt, and the foundation, framing and rough-ins are inspected
If you miss it: Unpermitted square footage is excluded from the appraisal and surfaces at sale
IRC R302.1
Fire separation distance from the lot line limits openings and requires rated construction
If you miss it: An addition pushed toward the side yard needs a rated wall or loses its windows

Provision	Limit	If you miss it
IRC R304.1	A habitable room is at least 70 sq ft, with no horizontal dimension under 7 ft	A narrow bump-out reads as a closet on the plan review and does not count as a bedroom
IRC R305.1	7 ft ceiling height in habitable rooms, measured to the lowest projection	A second story built inside an existing roofline runs short on height at the eaves
IRC R310.1	Emergency escape opening in every sleeping room, 5.7 sq ft net clear	A new bedroom whose only window is a small fixed unit fails and the opening has to be enlarged
IRC Table R301.5	Live loads the new floor must carry: 40 psf living areas, 30 psf sleeping rooms	A second story framed on the existing walls without an engineer's check overloads the foundation below
IRC R105.2	Additions are never permit exempt, and the foundation, framing and rough-ins are inspected	Unpermitted square footage is excluded from the appraisal and surfaces at sale
IRC R302.1	Fire separation distance from the lot line limits openings and requires rated construction	An addition pushed toward the side yard needs a rated wall or loses its windows

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Budgeting from a per square foot number alone; the foundation, the bath and the roof tie-in are where additions actually break the budget.
Assuming the existing furnace can heat the new space. Extending ducts into an addition often drops pressure everywhere else in the house.
Skipping the survey. Setbacks, easements and lot coverage limits can move or shrink the addition after the plans are drawn.
Forgetting the electrical service. A 100 A panel with a kitchen or a second HVAC unit added usually needs an upgrade to 200 A.
Matching siding and roofing exactly is rarely possible on an older house; plan for a transition detail or residing one elevation.
No contingency on a house over 40 years old. Opening a wall finds knob and tube, undersized headers or a rotted band joist often enough to plan for it.

Key facts

A garage conversion is usually the cheapest added square footage in residential work because the foundation, walls and roof already exist.
Small additions cost more per square foot than large ones: the foundation, the roof tie-in, the permit and the crew mobilization do not scale down.
A bump-out under about 24 in deep can cantilever off the existing floor framing, which removes the foundation line entirely.
Soft costs, meaning design, engineering, survey and permits, typically run 8 to 15 percent of construction on a residential addition.
IRC R304.1 requires a habitable room to be at least 70 square feet, and R305.1 requires a 7 ft ceiling height.

Frequently asked questions

How much does a 400 square foot addition cost?

A single story 400 sq ft addition on a crawlspace typically runs $112,000 to $241,700 all in, which is $280 to $604 per square foot once soft costs and contingency are included. A garage conversion of the same size costs far less because the shell exists.

Is a second story cheaper than building out?

Not usually. It avoids a new foundation, but the existing foundation, walls and framing have to carry the new load, which often means an engineer, new footings under posts, and moving out during construction.

What are soft costs on an addition?

Design, structural engineering, survey, plan review and permit fees, typically 8 to 15 percent of construction. They are spent before the build starts and are not refundable if the project does not go ahead.

Why do small additions cost so much per square foot?

The fixed work does not shrink. Mobilization, the roof tie-in, the foundation, the permit and the systems connection cost nearly the same on 200 square feet as on 600, so they are spread over fewer feet.

Sources and references

Next steps for this project

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