Home Addition Cost Calculator

Home addition cost calculator formula

A home addition cost calculator works from two rates and one area. The formula is total cost = footprint in square feet × (material rate + labor rate).

The calculator shows the material cost and the labor cost separately, then adds them. It does not know your local prices. The default rates of $150 material and $150 labor per square foot are placeholders, so replace both with figures from local quotes.

Worked examples

Example 1 is a bedroom addition, 20 ft × 15 ft, at $150 material and $150 labor per square foot.

• Footprint: 20 × 15 = 300 sq ft
• Material: 300 × $150 = $45,000
• Labor: 300 × $150 = $45,000
• Total: $90,000

Example 2 is a smaller family-room bump-out, 24 ft × 16 ft, at $120 material and $110 labor.

• Footprint: 24 × 16 = 384 sq ft
• Material: 384 × $120 = $46,080
• Labor: 384 × $110 = $42,240
• Total: $88,320

The second room is 28 percent larger and costs less because the rates are lower. That shows how much the rate matters. A $30 swing in either rate moves the 300 sq ft room by $9,000.

What the two rates should cover

The material and labor rates are doing a lot of work, so decide what goes in them before you type a number.

Part of the project	Usually in the rate?
Foundation or piers	Often yes, if you want an all-in figure
Framing, sheathing, roof	Yes
Windows, doors, siding	Yes, at the quality level you pick
Electrical, plumbing, HVAC	Yes for the new room, and extra if the house systems need upgrades
Insulation and drywall	Yes
Flooring, paint, trim	Depends on your finish level
Kitchen or bath fixtures	Usually extra, since they cost far more than a bedroom
Permits, design, engineering	Usually separate

A bedroom addition and a kitchen addition with the same footprint do not cost the same. Plumbing, cabinets and appliances make wet rooms the expensive ones. If your addition has a bathroom, price that part with a contractor and enter a higher blended rate.

How to get your rates

• Ask two or three local general contractors for a per-square-foot range on a project like yours. Ask what is and is not in it.
• Split the answer into material and labor if they will. If not, enter half and half and treat the total as the number that matters.
• Look at the finish level. Basic vinyl siding and carpet is a different job from fiber cement and hardwood.
• Check the site. Poor soil, a steep slope, or long trenches for utilities add cost that a plain rate will not show.

Add what the formula leaves out

Take the total from the calculator and add these before you plan a budget.

• Permit and plan review fees, which your local building department can quote.
• Architect, designer or engineer fees. Structural plans for a second story or a long span usually need an engineer.
• Utility work. Extending a gas line, a sewer lateral or an electrical service panel.
• A contingency. Opening an old house often finds problems. Many people hold back 10 to 20 percent for surprises.
• Temporary living costs if the work disturbs the kitchen or the only bathroom.

Common mistakes

• Using the house’s average cost per square foot. Kitchens and baths push the real figure up and big empty rooms bring it down.
• Measuring interior floor area instead of the footprint. Wall thickness adds area, and the footing and roof are bigger than the room.
• Forgetting the connection to the existing house. Matching roof lines, flashing and siding is real labor.
• Comparing quotes that cover different things. Get each bid in writing with the scope listed.
• Skipping the structural review for an addition over an existing floor. Always have a licensed professional check the loads.

Related calculators

The foundation repair cost calculator helps if the existing foundation needs work first. The framing cost calculator prices the structure alone, and the cost to rebuild a house calculator gives a whole-house comparison.