Building an in-law suite costs $25,000 to $265,000, averaging $82,750 on HomeAdvisor’s figures, with Angi publishing the same range and average (June and September 2026)23. HomeGuide frames it by project path: $5,000 to $100,000+ to convert existing space against $30,000 to $280,000 for a new addition or detached structure, at $100 to $350 per square foot (October 2025)1. Angi and HomeAdvisor are corporate siblings, and matching figures alone don’t establish independent corroboration — their pages match on most rows below, with HomeGuide providing the separate comparison. All figures are the latest published data from each source.
Cost by project type
The sibling pages publish an identical type table23; HomeGuide’s per-type figures diverge on every conversion row — ranges as published, no cause stated1:
| Project type | Angi / HomeAdvisor | HomeGuide |
|---|---|---|
| Converting a finished room | $10,150 – $65,625 | $5,000 – $100,000+ (any room) |
| Converting a garage | $20,000 – $60,000 | $5,000 – $50,000+ |
| Converting a basement | $10,000 – $30,000 | $7,500 – $52,000 |
| Converting an attic | $25,000 – $65,000 | $50,000 – $100,000+ |
| Extending a room (bump-out) | $26,600 – $54,800 | $85 – $200 per sq ft |
| Adding a room | $40,000 – $200,000 | $5,000 – $240,000 |
| Detached structure | — | $45,000 – $280,000 |
For detached options, HomeGuide prices an accessory dwelling unit at $100 to $300 per square foot ($30,000 to $240,000 total for 300 to 800 square feet), a garage with an apartment at $200 to $350 per square foot ($60,000 to $280,000), and a guest house at $150 to $250 per square foot ($45,000 to $200,000)1.
Cost by size
The siblings match at $100 to $200 per square foot including materials and labor, with a size table running to 2,000 square feet23. HomeGuide’s table covers new additions and detached structures only, at $100 to $350 per square foot1 — its per-foot ceiling sits above the siblings’ at every size.
| Square feet | Angi / HomeAdvisor | HomeGuide (new build only) |
|---|---|---|
| 300 | $30,000 – $60,000 | $30,000 – $105,000 |
| 400 | $40,000 – $80,000 | $40,000 – $140,000 |
| 500 | $50,000 – $100,000 | $50,000 – $175,000 |
| 600 | $60,000 – $120,000 | $60,000 – $210,000 |
| 750 | $75,000 – $150,000 | — |
| 1,000 | $100,000 – $200,000 | — |
| 2,000 | $200,000 – $400,000 | — |
Materials
The sibling pages publish an identical materials table — selected rows23:
| Material | Average cost |
|---|---|
| Framing | $15 – $32.50 per sq ft |
| Foundation | $4 – $16 per sq ft |
| Drywall | $4 – $6 per sq ft |
| Insulation | $0.30 – $1 per sq ft |
| Wiring | $2 – $4 per sq ft |
| Plumbing | $15 – $40 per sq ft |
| Flooring | $3.50 – $21 per sq ft |
| Windows | $100 – $1,500 each |
| Doors | $500 – $5,000 each |
HomeGuide’s separate factors table prices several rows differently: drywall at $1.50 to $3.50 per square foot, insulation at $1 to $4.50, plumbing at $4 to $5, and wiring at $4 to $9, plus cabinets at $150 to $500 per linear foot and appliances at $2,500 to $6,800 per kitchen1.
Labor and professional fees
Trade rates diverge across the sources: electricians run $50 to $100 per hour on the siblings against $50 to $130 on HomeGuide; plumbers run $150 to $275 per hour on the siblings against $75 to $150 on HomeGuide123. For general contractors, the siblings publish $300 to $500 per day plus $150 to $250 per day per additional laborer — or 10% to 20% of the total project — while HomeGuide publishes $50 to $150 per hour123. The siblings also price common task totals: electrical panel upgrade $1,600, rewiring $6,000, rough-in plumbing $6,500, sink installation $450, and water heater installation $1,40023. A new HVAC system for the suite costs $5,000 to $12,500 on the siblings, with HVAC pros at $100 to $250 per hour23.
Permits and requirements
Permit rules are set locally — confirm which apply with your city before budgeting. Angi’s published permit cost estimates, selected rows2:
| Permit type | Average cost |
|---|---|
| New construction | $150 – $2,000 |
| Garage or basement conversion | $1,200 – $2,000 |
| Electrical work | $10 – $500 |
| Plumbing work | $50 – $500 |
| HVAC installation | $250 – $400 |
| Roofing | $250 – $500 |
Two requirement-driven costs both HomeGuide and Angi call out: a basement suite generally needs an egress window meeting local code — HomeGuide prices installation at $6,000 to $8,000 — and a rented suite with tenant-paid utilities needs a separate electrical meter at $2,000 to $5,00012.
Value, rental income, and timeline
HomeGuide and Angi each state an in-law suite can raise property value by up to 30%, and Angi estimates $15,000 to $30,000 in potential annual rental income — these are the publishers’ broad estimates across projects, not budgeting predictions for an individual property: any value change depends on your market, and rental income depends on local rents, occupancy, and whether your municipality allows the rental at all12. On timing, HomeGuide and HomeAdvisor both put the project at 2 to 7 months — conversions around two months, new detached construction four to five months plus permitting13.
Related cost guides
See ADU cost for a fully independent backyard unit, garage conversion cost for the most common conversion path, and guest house cost for detached options.
Sources
- HomeGuide, How Much Does It Cost to Build a Mother-in-Law Suite? (2026), updated October 29, 2025
- Angi, How Much Does It Cost to Build an In-Law Suite?, updated September 21, 2026
- HomeAdvisor, What Is the Average Cost to Build an In-Law Suite?, updated June 17, 2026