Eli5: why do people say its cheaper to build up than out?

Question

I’ve heard contractors say this over and over again. It’s cheaper to build up when adding sqft to a home than to build an addition out? Is this true and if so why?

In: 1

Answer 1

Architect here. Almost entirely excavation and concrete costs. Most houses structure is strong enough to carry the weight going up – so it’s the exact same building, but out requires foundation, up doesn’t