What an addition costs by type and size — room, second storey, sunroom or garage conversion.
House Addition Cost Calculator
Inputs
Plumbing a new bathroom adds roughly $15,000–$35,000.
Adds 5–12% for drawings, engineering and permits.
Results
Estimated project cost
$63,000 – $134,400
area × $200–$400/ft² + 5–12% soft costs
Calculated with BuildCalculators.com
Calculated with BuildCalculators.com · buildcalculators.com/house-addition-cost-calculator
Estimates are for planning only — confirm quantities with your supplier or contractor before ordering.
An addition is priced per square foot of new floor area, and the range is wide because "addition" covers four quite different projects. Building a room out onto the garden means a new foundation, new walls, a new roof and a tie-in to the existing house. Going up means an engineer, a removed roof and a house you probably cannot live in. A sunroom is lighter construction. A garage conversion is mostly finishing a shell that already exists.
This calculator prices each of those separately, then adds the two lines that quotes routinely omit: a full bathroom if the plan has one, and the design, engineering and permit fees that arrive before a single nail does.
A = length × widthA × $200–$400/ft²A × $250–$500/ft²+5–12% for design, engineering, permitsA new house spreads mobilisation, design and site setup across thousands of square feet. An addition spreads the same overheads across a few hundred, then adds work a new build never faces: cutting into an existing roof and wall, matching siding and rooflines, protecting a house someone is living in, and working in a space with no room for a crew to spread out. Two hundred dollars a square foot is not a premium — it is the real arithmetic of small, complicated work.
Second storeys look cheaper on paper — no new foundation — and rarely are. The existing foundation and framing have to carry a load they were not designed for, which means an engineer, and usually reinforcement. The roof comes off, so the project runs on weather. And most families move out for the duration. Building out is the simpler project unless the lot genuinely has no space.
A garage conversion at $80–$200 per square foot is the best value in the table because the foundation, walls and roof are already standing. The work is insulation, a proper floor, heating and cooling, windows where the door was, and finishes. The same logic applies to basements and attics. The catch is parking, storage and — in some markets — resale value, since buyers do notice a missing garage.
Drawings, structural engineering, permit fees and sometimes a survey typically run 5–12% of construction. On a $90,000 addition that is $4,500–$10,800 spent before construction starts, and it is the line homeowners most often forget when comparing a builder’s quote against a budget they set themselves.
A single-storey family room extension, no bathroom.
$63,000 – $134,400
That is $210–$448 per square foot all in. Where you land depends mostly on finish level and whether the roof tie-in is simple.
Adding a full floor over an existing single-storey house.
$212,625 – $459,200
Before any of this: an engineer confirms the existing foundation and framing can carry it. That report can end the project or add a reinforcement line larger than the bathroom.
Turning an attached double garage into living space.
$33,600 – $89,600
Roughly half the cost of building the same 400 ft² new. Insulating the slab and getting heating right are what separate a converted garage from a room that always feels like a garage.
Installed, per square foot of new floor area, before soft costs.
| Type | Per ft² | 300 ft² project |
|---|---|---|
| Garage conversion | $80–$200 | $24,000–$60,000 |
| Sunroom, three-season | $100–$250 | $30,000–$75,000 |
| Room addition, single storey | $200–$400 | $60,000–$120,000 |
| Second-storey addition | $250–$500 | $75,000–$150,000 |
| Item | Typical cost |
|---|---|
| Full bathroom | $15,000–$35,000 |
| Kitchen | $25,000–$75,000 |
| Design and engineering | 3–8% of construction |
| Permits | $1,000–$5,000 typical residential |
| Foundation on difficult ground | +$10,000–$40,000 |
Additions are among the most variable projects in residential construction. Region alone can swing the rate by 50%, and finish level does the rest — the same 300 ft² room can be a $63,000 project or a $134,000 one without either number being wrong.
High-cost metros, complex rooflines, a foundation on sloping or poor ground, matching heritage or unusual siding, high-end windows and finishes, and any project where the family stays in the house and the crew works around them.
A simple rectangular footprint with a shed or gable roof that ties in cleanly. Keeping plumbing near existing stacks. Choosing standard window sizes. And deciding the scope before drawings are finished — mid-project changes are the single most reliable way to overrun a construction budget.
Hold 15–20% back on any addition. Opening up an existing wall reveals what is actually there: undersized framing, old wiring, rot at the sill, a foundation that stops short. None of it is visible when the quote is written, and all of it has to be dealt with.
The builder’s number is construction. Your budget also needs design, engineering, permits, appliances, window treatments and landscaping repair. Adding 15–25% to the construction figure gets you closer to the real project cost.
It saves a new foundation and buys an engineering assessment of the old one. That trade is often unfavourable — and the answer sometimes arrives after you have already paid for drawings.
Plumbing a new bathroom far from existing stacks means opening floors and possibly a new vent through the roof. Deciding it late turns a $15,000 line into a $35,000 one.
On anything structural — removing a load-bearing wall, building up, spanning a wide opening — the engineering is not the optional part. It is what makes the permit possible and the building safe, and retrofitting it after a failed inspection costs several times the fee.
A single-storey room addition runs $200–$400 per square foot installed in 2026 — about $60,000–$120,000 for a 20 × 15 ft room, before design fees. Adding a full bathroom pushes it up another $15,000–$35,000.
Because an addition is a small building with all the same trades and none of the economies of scale, plus the cost of tying into an existing structure — cutting into the roof and walls, matching finishes, and working around a house someone is living in. New builds spread mobilisation over thousands of square feet; an addition spreads it over a few hundred.
Rarely, despite needing no new foundation. A second storey runs $250–$500 per square foot because the existing foundation and framing must be verified — and often reinforced — by an engineer, the roof comes off, and the house is usually uninhabitable during the work. Building out is simpler unless the lot leaves no room.
Converting existing space. A garage conversion is $80–$200 per square foot because the foundation, walls and roof already exist — the work is insulation, flooring, heating, windows and replacing the garage door. Finishing a basement or attic is similar in principle. All are a fraction of new construction.
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It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.