Would I need planning permission for a 3m 1 story garden extension. Location KT3

At the back of the house it is permitted if if it is a semi detached or a detached house. However you must have mostly glazed walls, exterior grade doors between the extension and the house and it does not exceed 30 m2. This means it can be classed as a conservatory. This is called permitted development. There are some exceptions such as if you live in a Conservation Area or a National Park. If you don't wish for it to be glazed and want it to be open to the house you can use a Building Notice, but still won't need full planning. You will still have building inspectors and structural calculations, but the hassle of full planning will not be necessary.

Definitely contact your local building regs at the council. Get filled up on knowledge as best you can. The law changed some years back on planning permissions. I don’t live in your area so would check with your council. Better to be safe than sorry. A building engineer local to you will understand your property better. But it’s doable don’t worry.

I would say give the local planning officer or a fully qualified Architect a call to make 100% sure what you can or cannot do