Fact is, such an addition may not add to your property value at all. The market value comes into play only when you go to sell or refinance. How old will the unit be & in what condition at that time -- two, or five or ten years from now. Value will also make a difference if such an improvement is or is not  standard in the comparables that are chosen for the appraisal report. But, are you building a nice storage unit for market value or because this is what you need & will put to use in your long-term stay at the property?"