Home Addition Cost Calculator

Example Data Table

Examples are illustrative and can differ from local bids.

Formula Used

This calculator builds an estimate from an adjusted unit rate, then applies multipliers and soft-cost percentages.

• Direct Unit Rate = Base cost per sq ft + foundation allowance + finish allowance + (plumbing + electrical + HVAC allowances)
• Direct Cost = Area × Direct Unit Rate
• Adjusted Build Cost = Direct Cost × (Region × Complexity × Site Access × Quality × Stories)
• Subtotal = Adjusted Build Cost + Fixed Allowances (roof tie‑in, bathroom, kitchenette, demolition)
• Soft Costs = Subtotal × (Permits% + Design%)
• Waste = (Subtotal + Soft Costs) × Waste%
• Contingency = (Subtotal + Soft Costs + Waste) × Contingency%
• Total = Subtotal + Soft Costs + Waste + Contingency

How to Use This Calculator

1. Enter the new addition’s total square footage.
2. Select addition type and quality, then confirm the unit rate.
3. Choose region, complexity, access, and stories to reflect constraints.
4. Pick foundation and finish package, then add system allowances as needed.
5. Set permits, design, waste, and contingency to match your risk level.
6. Click Calculate to view totals and the cost breakdown.
7. Use the export buttons to download CSV or PDF for sharing.

Professional Guide: Planning a Home Addition Budget

1) Start with a reliable size baseline

Square footage drives most pricing. A 200–400 sq ft room addition often lands in the mid five figures, while 600–1,000 sq ft expansions can move into six figures quickly. Measure the footprint, confirm ceiling heights, and include closets, hallways, and stair landings in the total area.

2) Unit rate reflects scope, not just finishes

Cost per square foot varies by space type because systems differ. Bathrooms and kitchens need plumbing, waterproofing, cabinetry, and higher‑spec surfaces, so their unit rates are typically higher than a bedroom or living room. This calculator lets you choose an addition type and quality, then override the unit rate when you have local quotes.

3) Location and access shape labor efficiency

Regional pricing can shift totals by 10–25% or more depending on labor availability and material costs. Tight lots, limited driveway access, or restricted work hours add handling time and increase hauling and staging costs. Use the region and site‑access multipliers to reflect those practical constraints.

4) Complexity is the hidden multiplier

Complexity covers structural tie‑ins, roof integration, and utility routing. A simple bump‑out with straightforward framing differs from an addition requiring steel beams, long spans, or major roof rework. Selecting “Complex” or “Very Complex” increases the multiplier to represent higher coordination and longer installation times.

5) Foundation choices change the base cost

Slab construction is often the least expensive foundation approach, while crawlspaces, piers, and basements add excavation, formwork, waterproofing, and inspection steps. The foundation allowance in this calculator applies a per‑square‑foot adjustment so you can see how structural decisions affect the budget early.

6) Systems allowances prevent surprise change orders

Electrical, HVAC, and plumbing are frequent sources of overruns when omitted from early budgets. Even a bedroom may need a new circuit, additional outlets, lighting, and potentially ducting adjustments. Toggle the systems allowances to keep the estimate realistic, especially when extending or upsizing existing services.

7) Soft costs and risk buffers matter

Permits, inspections, and design/engineering are usually calculated as percentages because they scale with scope. Many homeowners also carry 3–5% waste/overage plus 10–15% contingency for unknown conditions in remodeling projects. Adjust these fields to match your risk tolerance and the project’s uncertainty.

8) Use the breakdown to compare options

After calculating, review the line items: adjusted build cost, fixed allowances (roof tie‑in, bathroom, kitchenette, demolition), and soft costs. Run multiple scenarios—changing finish package, complexity, or stories—to understand tradeoffs before requesting bids. Export CSV or PDF to share a consistent baseline with contractors and designers.

FAQs

1) How accurate is an online estimate?

It’s a planning tool, not a binding bid. Accuracy improves when your unit rate matches local pricing and you choose realistic multipliers. Use it to compare scenarios, then validate with contractor quotes.

2) What unit rate should I start with?

Start with the suggested rate for your addition type and quality, then adjust after collecting two or three local ballpark quotes. Higher‑system spaces like kitchens and bathrooms usually need higher unit rates.

3) Should I always include contingency?

Yes for most additions. Unknown conditions, code upgrades, and utility conflicts are common. A 10–15% contingency is typical for remodel‑style work; simpler new construction may use less.

4) Why does site access change cost?

Restricted access reduces productivity and increases labor hours. It can add costs for hand‑carrying materials, smaller deliveries, extra protection, and more trips for hauling and disposal.

5) When should I add a roof tie‑in allowance?

Add it when the new structure intersects existing roofing or requires valleys, flashing, or significant framing changes. Complex roof geometry and matching older materials generally increase the allowance.

6) Do permits and design costs vary by location?

Yes. Permit fees, required inspections, and engineering scope vary by municipality. Design costs also vary with complexity and the level of documentation needed for approvals and construction.

7) How do I use the export files?

Run your preferred scenario, then download CSV or PDF. Share the exported breakdown with contractors to keep assumptions consistent and to document scope changes across multiple estimate revisions.