Costs & permits
The pages that decide whether a project happens — sorted by the question you're trying to answer rather than which part of the house it touches.
Answer them in this order
Cost first, then whether it's legal on your lot. Financing and hiring only matter once those two check out.
What it costs
Real ranges and where the money goes.
What you can build
Permits, zoning, and legal classification.
What it returns
Whether the project shows up in an appraisal.
How to pay for it
HELOCs, refinancing, and renovation loans.
Who to hire
Vetting contractors and reading a real bid.
State-by-state permit rules aren't here yet.
Permit requirements are set city by city, and a national rule of thumb can't tell you whether your specific lot qualifies. Building that reference properly is the next major piece of work on this site — until it exists, the permit guide covers the general framework and tells you how to check your own jurisdiction.