The most expensive mistake in multiplex development isn't building too small — it's building too big. Here's the right-sizing math that decides whether the same lot makes money or loses it.
Zoning tells you the biggest building your lot allows. Almost everyone hears that number and wants to build it. Almost everyone is wrong — and the reason is a single sentence every owner should tattoo on their pro forma: rents scale with doors and bedrooms, not with square feet.
A tenant pays for a place their family fits: bedrooms, bathrooms, a real kitchen. A 3-bed/2-bath at 1,100 square feet and the same layout stretched to 1,500 square feet rent within a few percent of each other — the market prices the bedroom count, not the hallway width. Construction, meanwhile, is priced by the square foot, every single one.
So when a lot's envelope allows 7,900 sf and the standard five-door scheme needs 5,500, building to the envelope adds roughly 40% to cost and roughly 0% to income. We've watched that one decision turn a project from strongly positive to barely break-even on the same lot with the same rents:
| Right-sized — 5,500 sf | Envelope-max — 7,900 sf | |
|---|---|---|
| Units | 5 × 3bd/2ba @ ≈1,100 sf | 5 bigger units, same bedrooms |
| Build cost | ≈$1.75M | ≈$2.5M |
| Gross rent | ≈$187K/yr | ≈$195K/yr — barely more |
| Result | Loan covers the build; strong equity + cash flow | Same loan capacity, $750K more cost — the gap is yours |
When bigger IS right: compact lots that only fit smaller stacked 2-beds, sixplex-ward buildings where six leaner units beat five fat ones, and the rare street whose comps genuinely pay for luxury space. The rule isn't "always 1,100" — it's "let the rent roll size the building, never the zoning envelope." Envelope headroom isn't wasted; it's optionality you didn't pay for.
"Why this size?" If the answer is some version of "because the lot allows it," you're looking at a pro forma built backwards. The right answer starts from comparable rents per unit type on your street, works out the mix that maximizes income per construction dollar, and only then checks that zoning permits it. Size is an output of the math — never the starting point.
Free site read: doors, the rent-optimal unit mix, and the honest numbers both ways.
Get a Site ReadIllustrative figures at September 2026 construction and rent levels — planning estimates, not lender commitments or financial advice.