We almost built an addition. Got three quotes, drew up plans, sat through hours of contractor meetings, and ultimately decided not to do it. Not because it wasn’t worth it, but because the numbers made selling and buying elsewhere more sensible for our situation.
That research wasn’t wasted. I learned exactly what home addition costs look like from the inside, where the money goes, and why quotes can vary by $100,000 for the same project. If you’re considering adding space to your house, here’s what I wish someone had told me upfront.
What Home Additions Actually Cost
National averages sit around $150 per square foot for a basic addition, ranging up to $400+ per square foot for high-end finishes or complex builds. But averages hide more than they reveal.
A 400-square-foot addition at $150/sf costs $60,000. At $400/sf, that same footage costs $160,000. Both numbers are “accurate” for different projects.
Here’s how the ranges break down by type:
Room addition (bedroom, living space)
- Basic: $80-150 per square foot
- Mid-range: $150-250 per square foot
- High-end: $250-400 per square foot
Bathroom addition
- Half bath: $15,000-35,000
- Full bath: $35,000-75,000
- Primary suite with bath: $60,000-150,000
Kitchen addition or expansion
- Basic: $50,000-100,000
- Mid-range: $100,000-175,000
- High-end: $175,000-300,000+
Second story addition
- Basic: $100-200 per square foot
- Mid-range: $200-350 per square foot
- High-end: $350-500 per square foot
Sunroom or four-season room
- Basic prefab: $25,000-50,000
- Custom built: $50,000-120,000
In-law suite or ADU
- Detached: $80,000-200,000
- Attached: $60,000-150,000
These ranges assume you’re in a market with moderate construction costs. Coastal California, New York City, and similar high-cost areas will run 30-50% higher. Rural midwest or south might be 20-30% lower.
Why Costs Vary So Dramatically
Two 500-square-foot additions can cost $75,000 and $200,000. Here’s what creates that difference:
Foundation work
If your addition sits on a slab, foundation costs are minimal. If it needs a basement or crawl space, add $20,000-50,000. If your site has drainage issues, soil problems, or slope challenges, the foundation alone can exceed $30,000.
Roofing integration
A bump-out under existing roofline costs less than a full new roof section. Tying a new roofline into an existing one, especially on a second story, involves complex structural work and exterior matching that adds significant cost.
Utility connections
Adding a simple bedroom means running electrical and possibly HVAC ducts. Adding a bathroom or kitchen means plumbing, additional electrical, possible gas lines, and potentially upgrading your main service panel or water heater. Utility work can range from $5,000 for basic electrical to $40,000+ for comprehensive plumbing and mechanical.
Exterior matching
Making your addition look like it was always part of the house costs more than obvious add-on construction. Matching siding, roofing, windows, and trim to your existing house, especially if your house has older or discontinued materials, increases costs substantially.
Interior finishes
Builder-grade flooring, paint, and fixtures might cost $15-25 per square foot for finishes. Hardwood floors, custom millwork, and premium fixtures can push that to $50-100 per square foot.
Complexity of design
Simple rectangular additions with standard 8-foot ceilings cost less than designs with vaulted ceilings, bump-outs, bay windows, or irregular floor plans.
Permits and regulations
Building permits typically run $1,000-5,000 depending on project size and location. But some areas have impact fees, school fees, or development fees that can add thousands more. Some jurisdictions have lengthy approval processes that delay the project and add soft costs.
Where the Money Actually Goes
Here’s a rough breakdown of how a typical addition budget distributes:
Labor: 30-40%
Construction labor is the single largest expense. This includes framing, electrical, plumbing, HVAC, drywall, painting, and finish work.
Materials: 25-35%
Lumber, concrete, roofing, insulation, windows, doors, and all the components that make up the structure.
Interior finishes: 15-25%
Flooring, cabinets (if applicable), fixtures, appliances, paint, trim, and anything that finishes the interior.
Mechanical systems: 10-20%
Electrical panel work, HVAC extension or new unit, plumbing runs, and any specialty systems.
Foundation and structural: 10-15%
Foundation work, framing, structural engineering, and connections to existing structure.
Permits and design: 3-8%
Architectural drawings, engineering, permits, and inspections.
Contingency: 10-15%
Smart budgeting includes money for surprises. You’ll need it.
Our Addition Story (Why We Didn’t Build)
We wanted to add 400 square feet: a primary bedroom suite with bathroom. Our house is 1,650 square feet, and with two kids getting bigger, we’d outgrown the master bedroom closet-as-office situation.
First quote came in at $145,000. Second at $178,000. Third at $127,000.
That lowest quote concerned me. We dug into the details and found the contractor had underspecified the foundation work and assumed we’d handle permitting ourselves.
The highest quote included extensive landscaping restoration, premium windows, and a full HVAC zone for the addition.
The middle quote was most realistic for what we wanted: solid construction, mid-grade finishes, proper permitting handled by the contractor.
Then we did the math on alternatives:
Scenario 1: Build the addition
$165,000 estimated (middle quote plus 15% contingency)
3-5 months of construction disruption
End result: 2,050 square foot home, same neighborhood
Scenario 2: Sell and buy
Our house value: ~$420,000
Houses with the square footage we wanted: $475,000-550,000 in comparable neighborhoods
Net cost to move: $55,000-130,000 plus moving stress, but immediate result
Scenario 3: Renovate existing space
Convert underutilized dining room to primary suite: ~$45,000
Less square footage gained, but much less cost and disruption
We went with Scenario 3. Converted the formal dining room nobody used into a primary bedroom, added a bathroom in what was a big closet, and reclaimed the old bedroom as an office. $52,000 total, six weeks of work, and we love it.
The addition would have been great too. But for our situation, maximizing existing space made more sense than adding new.
Making the Decision: Add On or Move On?
Questions to ask yourself:
Do you love your location?
If your neighborhood, school district, or proximity to work is irreplaceable, building makes more sense than moving. If you’d be equally happy elsewhere, compare the full costs.
How long will you stay?
You typically recoup 50-70% of addition costs in home value. If you’re staying 10+ years, the value is in living there, not ROI. If you might move in 3-5 years, you’ll likely lose money compared to the alternative.
Is your lot suitable?
Setback requirements, lot coverage limits, and neighborhood covenants can limit what you can build. Some additions simply aren’t feasible on certain properties.
Can you handle the disruption?
Living in a construction zone for 3-6 months takes a toll. If you have young kids, work from home, or have health concerns, consider whether you can realistically manage.
What’s your financing situation?
Home equity loans, HELOCs, and construction loans all have different terms and requirements. Compare the cost of financing an addition versus a potentially larger mortgage on a different house.
How to Get Accurate Quotes
When we sought bids, I learned that contractor quotes are only as good as the information you provide. Vague requests get vague bids.
Before contacting contractors:
- Know roughly what you want (size, function, must-haves)
- Research zoning and building codes for feasibility
- Have your property surveyed if you don’t have a recent survey
- Decide on finish levels (basic, mid-grade, premium)
When requesting quotes:
- Get at least three bids from licensed, insured contractors
- Provide the same scope of work to each contractor
- Ask for itemized bids, not just lump sums
- Specify exactly what’s included (permits, cleanup, landscaping restoration)
- Ask about their current workload and realistic start dates
Evaluate quotes by:
- Total cost, obviously
- Itemization and transparency
- What’s included versus excluded
- Contractor’s communication style
- Timeline estimates
- References from similar projects
- How they handle allowances (fixed versus open-ended)
Red flags:
- Asking for large upfront payment (more than 20-30%)
- Unwillingness to provide detailed breakdown
- No physical business address
- Pushing to start immediately
- Significantly lower than other bids without clear reason
Timeline Expectations
Home additions take longer than you expect:
Design and permitting: 2-6 months
Architectural drawings, engineering review, permit submission, and approval can take months before any construction begins.
Foundation: 2-4 weeks
Excavation, forms, pouring, and curing the foundation.
Framing: 2-4 weeks
Building the skeleton of your addition.
Mechanical rough-in: 2-3 weeks
Electrical, plumbing, and HVAC before walls close up.
Insulation and drywall: 2-3 weeks
Weather-tight building envelope, insulation, and interior wall surfaces.
Finishes: 3-6 weeks
Painting, flooring, cabinets, fixtures, trim, and all the details.
Total timeline: 4-9 months from permit submission to move-in
Add time for weather delays, material shortages, and the inevitable surprises. Our contractor friends say to take the estimated timeline and add 30%.
Alternatives to Full Additions
If a major addition isn’t feasible, consider:
Bump-outs
Small extensions (2-4 feet) can add significant usable space to kitchens, bathrooms, or bedrooms without the full cost of an addition. Typically $15,000-50,000.
Basement finishing
If you have an unfinished basement, converting it to living space costs $30-75 per square foot, much less than above-ground additions.
Attic conversion
Underutilized attic space can become bedrooms or offices for $50,000-150,000 depending on structural requirements and dormers needed.
Garage conversion
Converting attached garage to living space typically costs $30,000-80,000. You lose parking and storage, but gain heated square footage without new foundation.
Prefab ADUs
Detached accessory dwelling units can be delivered and installed for $80,000-200,000, often faster and with more predictable costs than stick-built additions.
Existing space reconfiguration
Sometimes the square footage you need already exists. Combining rooms, removing walls, or reimagining layout can create functionality without adding foundation.
Final Thoughts
A home addition is a significant investment, both financially and emotionally. It’s not the right choice for everyone, and it’s not the only way to get more space.
Before committing, run the full analysis: cost to build versus cost to buy elsewhere versus cost to optimize what you have. Factor in disruption, timeline, and your family’s capacity to manage a major construction project.
If an addition is the right choice, go in with clear expectations, detailed planning, and contingency funds. The difference between a good addition experience and a terrible one often comes down to preparation.
And maybe budget 20% extra. Just in case.
