You cannot build houses or hotels on any property in a color group if even one property in that group remains mortgaged.
You must pay the Bank the mortgage value plus 10% interest. Once paid, the deed is flipped face-up, and you can begin collecting rent or building houses (provided you own the full color group). buying a mortgaged property in monopoly
You must still pay the 10% interest fee to the Bank immediately. If you choose to lift the mortgage later in the game, you will be required to pay the mortgage value plus another 10% interest to the Bank. Key Strategic Constraints You cannot build houses or hotels on any