I have gone thru this process a few times for different flipper projects I investigated. The best thing to start with is the comparable homes in the area. Once you know the square footage and comp prices you have something to compare to.

Next thing, get a contractor who can give you a price on each thing you want to do(garage, baths, kitchen, flooring, etc).

Then do the math to see if the numbers work out. Pick and choose the ones that add the most value. Sometimes built ins and custom wood work you don't get your money back. Most kitchen and bath stuff you do get your money back, especially if they are more than 25 years old.

Also, it is helpful to have a real estate agent you trust and who has integrity. They are a key component in understanding the market value (unless you have been studying it and have a realistic idea).

Now is the time to do it. Go fast before the economy turns around in a year or two. Good luck.