Does anyone know what size a room has to be when you build an extendtion to a house, to be classed as another room.

isn't it to do with the function rather than the size.

I don't suppose it makes a lot of difference really, as if it came to selling the house, you described it as 5 bedroom, but one bedroom was only the size of a closet then buyers would just treat it as 4 bedroomed.

It would help if you said what you want to do because it varies as to what exactly you are intending to do. You can't just extend your living room by a couple of feet, but you can build a porch on your front door. A conservatory doesn't need planning (up to a certain size) providing it has a glass roof and external doors to go between it and the main house.

It sounds like you want to build a "habitable room", in which case you will need planning regardless of size.

What's your plan?