要不要装修地下室？

First, the basement space is not counted as the living space in your home appraisal.  If you add some bedrooms and bathrooms, it will add some value to your house.  Since it is not a living space, future buyers will not give you much value as your normal living space, it does not matter how nice your basement is.  

Adding a bedroom and a bathroom in the basement do add a lot of utility value to the home owner.  Personally, I don't think that it is worth of investing in the basement space.  Just remember, the basement is trouble, flooding, leaking, etc.