Re-roofing

If your roof needs replacing, why not see it as an opportunity to install a new room and maybe offset the cost against the increased value of the house?

If, for example, you go for a box dormer on one side then you only need "half a roof" to finish the job so you really do save money.

Or, if you don't want to go that far, you could opt for a Velux conversion like this one. Here we replaced an old sagging roof structure supported by ageing joists with a metal beam across the whole building. This created an empty hollow for a guest bedroom.