Or improving the bathroom or kitchen, improving the flow or layout of the property can all improve the value.
Even improving the garden can add value... just don't add a swimming pool.
Adding the loan to the mortgage is one of the best financing options to do this.
You can repay this over a shorter period of time than the remaining mortgage term.
Overall you will need to have a good credit rating (worth checking), and be able to afford the repayments.
HTH
Sparky.