Why a single per-square-foot number doesn't tell you much
Search "home addition cost per square foot" and you'll find a wide range of numbers, most of them national averages that don't account for Michigan's climate, soil conditions, or local permitting requirements. A per-square-foot figure can be a useful starting point for a rough budget conversation, but two additions of the same square footage can cost very differently depending on what's underneath them, what's tying into them, and what's inside them.
The honest answer is that we don't quote projects off a blanket per-square-foot rate, because it hides more than it reveals. A 300-square-foot family room addition on a slab with no plumbing is a fundamentally different project than a 300-square-foot second-story bedroom addition with a new bathroom. Both might get described the same way in a casual conversation, and both would price out very differently.
What we can do is walk through the variables that actually move the number, so you go into a conversation with a contractor knowing what to ask about instead of anchoring on a figure you saw online. The sections below cover the four biggest cost drivers we see on Metro Detroit addition projects: foundation type, story count, mechanical tie-ins, and finish level.
Foundation type is usually the biggest swing factor
Every addition needs a foundation, and what goes under your new square footage depends on your lot, your existing home's foundation, and what the space is used for. A slab-on-grade addition is generally the least involved foundation option, but Michigan's frost line means even a slab requires footings poured below frost depth, not just a poured pad. Skipping that step isn't a shortcut available in this climate.
A full basement extension under an addition costs more upfront than a crawl space or slab, because you're excavating, forming and pouring foundation walls, and typically adding a footing drain and sometimes a sump connection. But it also adds usable square footage below grade that a slab or crawl space doesn't, which is worth weighing against the upfront cost if you'd otherwise finish that space later anyway.
Matching the addition's foundation to the existing home's foundation height and structure also matters more than people expect. If your existing home sits on a basement and the addition goes on a slab, you'll have a step-down or a structural transition to manage, which can add cost in framing and waterproofing detailing at that seam.
Because soil conditions and frost depth requirements vary by municipality and even by lot, the right foundation approach for your addition is something we determine during a site visit, not something we can generalize from a blog post. This is exactly the kind of variable that makes a flat per-square-foot number unreliable.
Second-story vs. ground-floor additions
Building up instead of out changes the cost profile in both directions. A second-story addition avoids the cost of a new foundation footprint since it typically sits on the existing structure, but it usually requires structural engineering to confirm the existing walls and footings can carry the added load, and often requires temporary roof removal and staging that affects how livable the rest of the house is during construction.
A ground-floor addition needs its own foundation and typically its own roof tie-in to the existing structure, which adds cost in a different place — excavation, foundation work, and roofline flashing and siding matching. It generally has less structural engineering complexity than adding a full story, since you're not loading an existing wall system.
Which direction makes more sense for your home depends on your lot size, setback constraints from your municipality's zoning ordinance, your existing foundation's condition, and your budget. We cover this tradeoff in more depth in our second-story versus ground-floor addition guide, but the short version is that neither direction is categorically cheaper — it depends on your specific house.
Mechanical tie-ins add up fast
Extending electrical, plumbing, and HVAC into a new space is rarely as simple as running a few extra wires and pipes. If your addition includes a bathroom or kitchenette, you're looking at new supply and drain lines, which may require tying into existing stacks in ways that involve opening up finished walls or floors elsewhere in the house.
HVAC is its own conversation. Many older Metro Detroit homes have a single furnace and duct system sized for the original footprint, and simply extending ductwork into an addition can leave both the old space and the new space poorly conditioned. Depending on the addition's size and location, that might mean a dedicated zone, a mini-split, or in some cases a second system — each with a different cost.
Because Qualified Construction keeps licensed electrical and plumbing trades in-house rather than subcontracting them out, mechanical tie-ins are scoped and priced as part of one coordinated plan rather than as separate bids from separate companies. That doesn't make the work free, but it does mean fewer surprises about how the trades interact with each other.
Finish level: where budgets actually get decided
Once the shell, foundation, and mechanicals are accounted for, finish level is where a lot of the remaining cost lives — and where homeowners have the most control. Flooring, cabinetry, countertops, trim detail, and fixture selections can move the finished cost of a comparably sized space substantially, independent of the structural scope.
This is also the area where it's easiest to get a number that sounds good on the phone and then balloons once selections are made. A fixed-price contract only holds if the finish-level assumptions are locked in before the contract is signed, which is why our estimates walk through allowances for these selections rather than leaving them as a vague line item.
If you're trying to set a realistic budget before you've picked a single tile or fixture, it's reasonable to plan for a range and firm it up once you've walked a showroom or two. What we can tell you with more confidence than a generic per-square-foot number is how your specific foundation, story count, and mechanical scope compare to similar projects we've built — which is what a free on-site inspection is for.
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