When a house is almost right, the question is whether to add the missing space or buy it. At today’s rates the math has shifted toward staying put more often than it used to — but only after one check.
Run the zoning check first
Before any cost comparison, confirm your lot can legally hold the addition: setbacks, lot coverage, and height limits. A plan that can’t be permitted isn’t a cheaper option, it’s a dead end.
The cost comparison
Compare the all-in cost per square foot of the addition against the cost of buying up — including the transaction costs of selling and the rate on new financing. When staying put wins, it usually wins by a wide margin.