A room addition costs $200 to $500 per square foot, a detached ADU runs $150,000 to $400,000, and a three-season sunroom starts at $80 per square foot. The right choice depends on the problem: everyday living space calls for conditioned square footage, while a sunroom only makes sense if you want a true three-season room.
How do the four main addition types compare on cost?
| Addition Type | Typical Cost | Best For |
|---|---|---|
| Bump-out (2-4 ft) | $4,000 to $30,000 | Stealing space for a nook or closet, often no new foundation |
| Full room addition | $200 to $500 per sq ft | Bedrooms, baths, kitchens; touches foundation to finishes |
| Detached ADU | $150,000 to $400,000 | Rental income ($1,200-$2,500/month) or aging-in-place space |
| Three-season sunroom | $80 to $230 per sq ft | Daylight and seasonal living without full HVAC |
| Four-season sunroom | $200 to $400 per sq ft | Only worth it if built to full house insulation standards |
A full addition costs the most per foot because it touches everything: foundation, framing, roof, electrical, plumbing, and HVAC. A bump-out often borrows an existing wall and roof overhang, so its marginal cost stays low. A detached ADU skips tying into the main house but pays for its own foundation, roof, and utility hookups, giving it the highest fixed startup cost.
Return on investment varies just as sharply. A room addition or finished ADU commonly recovers 50% to 70% of cost at resale, while a bump-out often returns more in livability than dollars. A four-season sunroom is the gamble: appraisers typically count it as living space only if it's heated and cooled like the rest of the house, so a half-insulated version can cost like a real room and appraise like a porch.
Match the type to the actual problem. Want a bigger primary suite? Build conditioned space. Want rental income or space for a parent? Only an ADU pays you back monthly. Want daylight and a spring reading nook? A three-season sunroom does that for a third of a full addition's price.
What's in a 400 sq ft room addition budget?
Here's a realistic line-item breakdown for a conditioned 400 sq ft addition in a mid-cost US market:
- Design, engineering, and permits: $6,000 to $18,000, higher if a structural engineer is needed for the roof tie-in.
- Foundation and slab or crawlspace: $10,000 to $30,000, depending on soil and frost depth.
- Framing, roof, and weather-tight shell: $30,000 to $60,000.
- Electrical, plumbing rough-in, and HVAC extension: $15,000 to $35,000.
- Interior finishes, flooring, and paint: $20,000 to $45,000.
Those finish lines overlap with standalone projects like an interior paint estimate and a flooring installation budget. Pricing each trade separately keeps a contractor's blended number honest.
Two cost drivers routinely surprise homeowners. Extending HVAC into 400 new square feet can overwhelm an existing furnace or heat pump, forcing a $5,000 to $12,000 system upgrade nobody quoted. The connection point where new meets old also takes skilled labor that flat per-foot estimates ignore, so hold back a contingency of at least 15% of the total.
What mistakes should you avoid?
The biggest mistake is skipping the design phase and treating an addition like a cosmetic remodel. An addition changes load paths, drainage, and your roofline, so $6,000 to $18,000 spent on drawings up front prevents a $20,000 mid-build change order.
Homeowners also underestimate matching costs: new siding, a continuous roofline, and floors meeting at the same height all cost real money, and a mismatched addition can lower appraised value. Skipping a designer to save money often leads to layout errors that a few hours of professional time, priced like an interior designer consultation, would have caught. Confirm setback rules and ADU eligibility before you start; a stop-work order after the foundation is poured is the most expensive delay there is.
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Frequently Asked Questions
Does a home addition increase property taxes?
Yes. Conditioned square footage gets reassessed and raises your property tax bill, typically in proportion to the added living area. Budget for the higher annual bill, not just the construction cost, before you commit.
Is an ADU worth the high upfront cost?
An ADU is worth it when local rents support $1,200 to $2,500 a month and zoning permits a rental unit. In those markets, the unit can pay for itself within 8 to 12 years while adding lasting resale value.
How long does a typical room addition take?
A standard conditioned room addition runs three to five months from permit to final inspection, assuming no major weather or material delays. Design and permitting before construction can add another one to three months on its own.
Is a bump-out cheaper than a full addition?
Yes. A bump-out costs $4,000 to $30,000 total because it often skips a new foundation and borrows an existing wall and roofline. A full addition runs $200 to $500 per square foot since it touches the foundation, framing, and every major system.
