In order to carry out work under Permitted Development the building work must conform to specific criteria, however recent changes in legislation have allowed more scope for what is allowed under Permitted Development. (See: What is the difference between Planning Permission and Permitted Development?)
You can now extend a dwelling by 8m to the rear of the rear facade if it’s single storey construction or 6m if it’s double height.
There are height restrictions. Often this means a single storey extension not being higher than 4m in height to the ridge and the eaves, and ridge heights of any extension not being higher than the existing property.
Two storey extensions must not be closer than 7m to the rear boundary.
It must be built in the same or similar material to the existing dwelling.
Extensions must not go forward of the building line of the original dwelling.
Side extensions must be single storey, maximum height of 4m and a width of no more than half of the original building.
In Designated Areas, side extensions require planning permission and all rear extensions must be single storey.
An extension must not result in more than half the garden being covered.
You can only do it once and the original building is either as it was on 1st July 1948 or when it was built.