Most people make a decision before they even go inside. Do invest in central heating. It costs about 2,000 for the average three-bedroomed home.
Families find a room that they can eat and work in very handy.
Do install a fully-fitted bathroom. You can have the full monty, complete with gold taps, for 1,000 upwards. Spend the most you can afford. Keep the room classic white and don't go for anything faddy. Do get a conservatory.