There is definitely a value to some home improvements. However, not all home improvements are equal. Overall, the fair market value of your home will be determined by you and the person buying your home. Once you agree on a price, the buyers bank will do an appraisal to be sure that their investment (aka the mortgage) is a safe one. They do this by sending over an appraiser to walk through and look at your home and then compare the specifics of your home to others that have sold in the area.
If you have concerns about your home appraising for the accepted offer price, there are a few things you can do. You can be sure your home is looking its best the day of the appraisal. Leave out a list of the improvements you've made to the home especially if they are not immediately apparent ... (keep in mind the appraiser has no obligation to take or even look at your list) ... and have your agent prepare a packet for the appraiser that includes the contract, listing, tax card and some comps. Again, the appraiser may or may not be willing to take or look at that information but it is always better to have that information together.