Home additions, from permit to final walkthrough

What a home addition actually involves

An addition is a new structure tied into an existing one, and that tie-in is where most of the risk lives. The work includes site prep and excavation, a new foundation (slab, crawlspace, or full basement depending on what matches the existing house), framing, roofing that ties into the existing roofline, exterior siding or brick to match, all new electrical and plumbing runs, insulation, drywall, and finish work. If the addition is a second story instead of a bump-out, the existing first-floor structure and foundation have to be evaluated for whether they can carry the additional load, which is a structural engineering question, not a guess.

The two addition types we see most in Macomb, Oakland, and Northern Wayne counties are single-story bump-outs (family rooms, primary suites, sunrooms, mudrooms) and second-story additions on ranch homes, which are common in Sterling Heights and Warren given how much of the housing stock there is single-level. In-law suites, whether attached or over a garage, are the third common request, and those carry their own code requirements around a second kitchen or kitchenette and a separate entrance if you want it to function as a legal accessory dwelling.

The foundation decision matters more than most homeowners expect. Matching a full basement under a new addition to tie into an existing basement is the most expensive option but avoids a difference in floor elevation and future settling issues. A slab-on-grade addition is cheaper and faster but only works well for single-story additions where you're not trying to match a basement level. We walk through this trade-off at the estimate stage with real numbers for both options, not just a default recommendation.

Realistic cost ranges for 2026

$200 to $400 per square foot is the honest range for a fully finished addition in this market, including foundation, framing, roofing, siding, electrical, plumbing, HVAC extension, insulation, drywall, flooring, and trim. A basic single-story bump-out without a bathroom or kitchen tends to land toward the lower end, around $200 to $275 per square foot. An addition that includes a full bathroom, kitchenette, or is built on a full basement foundation runs $300 to $400 per square foot because of the added plumbing, excavation, and mechanical work.

A 400 square foot addition, a common size for a family room or primary suite bump-out, runs roughly $80,000 to $160,000 all in. A 1,500 square foot second-story addition, which effectively doubles a ranch home's living space, runs $300,000 to $600,000 once you include the structural work to support the new floor, which is why second-story additions are priced separately from ground-level bump-outs rather than by a single blended rate.

HVAC is worth calling out on its own. Most additions need either an extension of the existing system or a dedicated mini-split or zoned unit, because the existing furnace and ductwork usually weren't sized for the added square footage. We size this at the estimate stage rather than assuming the existing system can absorb it, which is a common shortcut that leads to a cold or overworked addition later.

Timeline

A single-story bump-out addition typically runs 3 to 5 months from permit approval to final walkthrough, and that includes 2 to 4 weeks of Michigan weather-dependent foundation and framing work that can shift with a hard winter. A second-story addition runs 4 to 7 months because of the structural engineering, temporary roof removal, and the fact that most of the house is disrupted during the tie-in phase. Permit review itself typically adds 3 to 6 weeks before construction can start, and we submit those applications as early as possible so they're not sitting on the critical path.

Permits and setback rules in Macomb and Oakland counties

Every addition requires a building permit, and most require a zoning review for setback compliance, since townships and cities in Macomb and Oakland counties each set their own minimum distances from property lines. Sterling Heights, Rochester Hills, and Troy each have their own zoning ordinance and building department, and a addition that pushes closer to a side or rear property line than the local setback allows needs a variance application before a permit gets issued. Second-story additions and any structural work also require a stamped engineering plan in Michigan. We handle the zoning review, the permit application, the engineering coordination, and every inspection along the way.

What commonly goes wrong, and how it gets avoided

The most common problem on additions is a foundation tie-in that doesn't account for how the existing house has settled over decades. A new foundation poured without accounting for that mismatch can crack or separate from the original structure within a few years. Mike's inspection background means he's spent years evaluating exactly this kind of settling and differential movement in existing Metro Detroit homes, so the tie-in gets engineered around the house's actual condition, not a generic assumption.

The second common issue is an addition that changes how water drains around the house, creating a new low spot that floods a basement that never flooded before. Grading and drainage get planned as part of every addition, not treated as an afterthought once the framing is up.

What's included, what's not

Included: site prep, foundation, framing, roofing tie-in, exterior finish to match the existing home, all electrical and plumbing rough-in and finish, HVAC extension or new dedicated unit, insulation, drywall, flooring, and interior trim. Not included by default: interior furnishings and appliances, landscaping restoration beyond basic grading, and structural engineering fees for the stamped plan, which we quote separately since the fee depends on the scope of the addition.