Room Addition Cost Calculator
Estimate a room or home addition from its footprint and the $/sq ft you enter, plus any line add-ons.
Calculator
A 240 sq ft addition at $200.00/sq ft is about $48,000.00. Additions are structural and MEP work — they need licensed pros, a permit and inspections.
A room addition — a bump-out, a new bedroom, a family room, a second story — is most easily estimated on a cost-per-square-foot basis: multiply the new footprint by an all-in build rate, then add any extras that rate does not cover. Additions carry a higher $/sq ft than interior finishing because you are building foundation, walls, roof and systems from scratch.
The rate is the number you enter from a real quote, so the estimate reflects your market and your finish level rather than a national average that ages.
Because additions build new structure and tie into existing systems, the achievable $/sq ft depends on the type of addition (a single-story bump-out, a full room, a second story), how much the roofline and existing walls have to change, and your finish level. Estimating on a rate you supply keeps those variables in your hands and the arithmetic honest, whatever the market is doing.
Formula
Footprint times your rate, plus extras:
total = area (sq ft) × your $/sq ft + line add-ons
The $/sq ft bundles foundation, framing, roofing, siding, windows, and the electrical/plumbing/HVAC tie-ins for a basic finished room. Put anything outside that — a high-end kitchenette, extensive site work, an upstairs bathroom — into line add-ons so nothing is double-counted.
Worked example
For a 240 sq ft addition at an all-in $200/sq ft with no extra add-ons:
240 × $200 = $48,000
So the build is about $48,000. A larger footprint or a higher finish rate scales it directly — 400 sq ft at $250/sq ft would be $100,000 before add-ons. To convert any quote into a $/sq ft you can compare, use the cost per square foot tool.
Why additions cost more per square foot
Unlike finishing existing space, an addition builds the whole envelope: foundation, framing, roof, siding and windows, then ties new electrical, plumbing and HVAC into the house. That is why the per-square-foot rate is well above interior finish work, and why a second-story addition — which reworks the existing roof and structure — costs more than a ground-level bump-out.
Additions are structural and mechanical work by definition: they require licensed pros, a building permit and inspections, and often a survey or zoning check. Confirm setbacks and code with your local building department, and price the job from itemized written quotes.
Frequently asked questions
How much does a room addition cost per square foot?
It ranges widely by region, finish level and complexity, so the tool uses the all-in $/sq ft you enter rather than a stored figure. Multiply your footprint by that rate; a second story or complex site raises it, a simple ground-level bump-out lowers it.
How do I estimate a 240 sq ft addition?
Multiply the footprint by your build rate: 240 sq ft at $200/sq ft is about $48,000. Add any extras the rate excludes as line add-ons. Use the rate from a real quote so the estimate reflects your market.
What do the line add-ons cover?
Anything not baked into your $/sq ft: a kitchenette or bathroom fit-out, high-end finishes, extensive site or foundation work, or long utility runs. Keeping them separate avoids double-counting and makes the base rate easy to compare across quotes.
Do room additions need a permit?
Yes. Additions are structural and mechanical work that require permits and inspections, and they must respect zoning setbacks. Confirm requirements with your local building department before you start, and use licensed professionals for the structural, electrical and plumbing scope.
Is adding on cheaper than a whole-house renovation?
They answer different questions: an addition prices new square footage, while a whole-house renovation reworks what exists. Estimate the scope you actually plan, and compare on a $/sq ft basis so the numbers are apples to apples.
Does a second-story addition cost more than a ground-level one?
Usually, yes. A second story reworks the existing roof, reinforces the structure below and complicates access and staging, so its $/sq ft runs above a simple ground-level bump-out on a new slab. Reflect that by entering a higher rate for a second-story build than for a single-story addition.