An extra room on my house. It would give me a lovely large sunny room with plenty of bookshelves, a view of the sea, a woodstove, an en suite with underfloor heating for the shower, and a sun terrace outside the folding doors. It would connect the house with the garden (currently on very different levels). I've got the planning permission, I've got the money, I've got a builder lined up for next year. But it is absolutely a want not a need, it is expensive (access issues) and it would be much more sensible to move a septic tank and build a garage. So, stupid. Which doesn't stop me dreaming of it.