A room addition can change the way a house works for a family. It can also expose a homeowner to bigger expenses than a simple price-per-square-foot estimate suggests. Here is how to put the numbers into a usable plan.
2026 price overview
Angi’s 2026 U.S. cost guide estimates $80–$200 per square foot for some ground-floor additions, while more complicated additions, especially projects that build upward, may cost significantly more. The source also reports an average total project cost around $51,000, but averages can be misleading because project sizes are so different. A short bump-out and a new second story should never be budgeted as if they were the same job.
Before comparing anyone’s quote, define exactly what you are purchasing: gross added floor area, finished living area or renovation of existing space. A contractor may count only the new square footage, but the project may require work to existing rooms, the HVAC system or the roof.
What different additions can cost
These are the editable planning rates used by our calculators, not regional market quotes. They are drawn as illustrative bands around published national guides:
| Project | Planning band per sq ft |
|---|---|
| Bedroom / living area | $120–$250 |
| Bathroom addition | $200–$425 |
| Kitchen extension | $220–$450 |
| Sunroom | $100–$325 |
| Attached garage | $85–$185 |
| Second story | $200–$500 |
Wet rooms cost more because plumbing, drainage, waterproofing, ventilation and fixtures add complexity. A second story can require structural reinforcement and modifications to the occupied home. A sunroom designed for year-round use can be very different from a three-season enclosure.
Five costs that get missed
1. Architectural and engineering design
Plans may need to demonstrate structural safety, local energy requirements, egress, stormwater management or site setbacks. A contractor’s initial construction-only number may not cover those professional services.
2. Permits and local fees
Building departments assess permit and inspection fees differently. Some projects trigger utility, school, impact or connection fees. Check your city or county schedule before treating a generic estimate as final.
3. Work on the existing home
A new addition may require temporary protection, demolition at the connection point, a new electrical panel, modified ductwork or matching the exterior cladding and interior finishes.
4. Site conditions
Access limits, poor soils, trees, slopes, drainage, old foundations and buried utilities can turn a straightforward idea into a complicated project.
5. Contingency
Planning for a margin of uncertainty is sensible. Our calculator begins with a 15% contingency assumption, which you can adjust. This is a model setting, not a claim that every project requires exactly 15%.
Example: a 400-square-foot bedroom addition
For a standard-finish 400 sq ft bedroom at a typical U.S. planning scenario, our example baseline of $120–$250 per sq ft yields $48,000–$100,000 in construction. An illustrative 5%–10% allowance adds $2,400–$10,000 for design and permits. With a 15% contingency on that subtotal, the all-in planning range becomes approximately $57,960–$126,500. If you select additional plumbing, unusually difficult site conditions or a major HVAC upgrade, add those items rather than hiding them inside a low headline estimate.
What to request before hiring
Ask at least three appropriate local contractors to review the same scope. Each written proposal should explain demolition, foundation and framing, roof connection, windows, insulation, systems, interior finishes, permits, allowances, change orders, start dates and payment milestones. Check licensing and insurance against your local requirements.
You can use our contractor bid comparison to bring extra allowances and omitted work into the same view. If the contractor cannot explain a large difference, do not assume the smaller total represents the same finished project.
Sources and further reading
- Angi (2026), How Much Does a Home Addition Cost?
- Angi (2026), How Much Does an ADU Cost to Build?
- Angi (2026), How Much Does It Cost to Add a Second Story?
Related cost guides
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