No. 5: 2-story addition
This project is a twofer: It adds a family room downstairs, plus a new bedroom and bathroom upstairs.
"If you really need some square footage, I think it's a good idea," says Combs.
But, depending on where you are, you may have some less expensive options for enlarging your home -- such as finishing off a basement, she says.
One pro tip: Move out for the duration of the renovation, says Mike Holmes, contractor and host of HGTV's "Holmes on Homes."
In the first place, a two-story or second-story renovation affects too many parts of the house, so living there is often not a good idea, he says. "It's a nightmare to live in."
"Too many times, contractors say you can live in the house" while they do the work. But you really can't, Holmes says.
It can be so stressful that he's dubbed the gritty byproduct of large-scale renovations "divorce dust."
Holmes' advice: Relocate people, pets and belongings. But arrange to be present every time an inspector is there "so that you can hear what he says about the work."