How to know if a spec home will be profitable

The formula

Profit = ARV − (land + hard costs + soft costs + cost of money + selling costs)

• ARV — what the finished house will sell for, from recent comparable sales nearby (similar size, age and finishes), not from the listing prices.
• Hard costs — the construction itself: site work, foundation, framing, systems, finishes.
• Soft costs — permits, impact fees, plans, engineering, surveys, insurance.
• Cost of money — interest and points on the land and construction loans. On a construction loan interest grows as draws are funded, so the schedule matters.
• Selling costs — the real estate commission and the closing costs you pay as the seller. Leaving out closing costs is one of the most common mistakes.

A worked example

Whether 12.5% is enough depends on your market, your financing and how much can go wrong. A common rule of thumb among spec builders is to look for roughly 15–20% on the sale price, but the right target is the one that covers your risk.

Work backwards: the most you should pay for the lot

Maximum lot price = ARV × (1 − target margin) − (hard + soft + cost of money + selling costs)

With the example above and a 15% target margin, the most you could pay for the land is about $105,000 — versus the $120,000 asking price. This is the number to bring to the negotiation.

Where spec home math usually goes wrong

• ARV taken from listing prices instead of closed sales.
• Selling costs counted as the commission only, without closing costs.
• Interest calculated on the full loan for the whole term — or not at all — instead of on the drawn balance.
• No contingency for overruns and delays.
• Comparing the per-square-foot cost against a number that includes the land.