Home Addition Cost in Metro Detroit (2026)

Three things move an addition’s price more than anything else. Foundation comes first: a slab-on-grade bump-out is cheap next to digging a full basement or matching an existing foundation depth. Roof tie-in is second, because marrying a new roofline into the old one is skilled, weather-critical work, and a second-story addition means opening up and reinforcing what’s already there. Matching the existing house is the quiet budget-killer: sourcing siding, brick, and windows that blend with a home built decades ago, then detailing the seams so the addition disappears into the original. Add plumbing or a kitchen and the per-square-foot number climbs again, since those rooms carry the most expensive systems.

• Sunroom (three-season): $150 to $250 / sq ft
• Ground-floor room addition: $200 to $350 / sq ft
• Bump-out (a few feet off one room): $250 to $400 / sq ft, since the tie-in cost spreads over less space
• Second-story addition: $250 to $400+ / sq ft, plus temporary weatherproofing and structural reinforcement below
• In-law suite or ADU with kitchen and bath: $300 to $400+ / sq ft

Design, engineering, and permit fees are usually a separate line and worth budgeting for up front.

Additions are where vague quotes hide the most risk, because so much of the cost is underground or inside the walls. Insist that the quote names the foundation type, spells out the roof and structural tie-in, and lists an allowance for matching siding and windows rather than burying it. A per-square-foot number with no scope is where change orders are born. When we match you with an addition contractor, you get an itemized, permit-ready quote you can actually compare against another, instead of guessing what each builder left out.