Under Permitted Development rules, the depth of your single-storey extension can be up to 8m using the neighbourhood consultation scheme. Factors impacting on both the allowed width and depth of the extension is the right of light to neighbouring properties and whether your property is detached. More detailed information on the allowable depth of your single-storey extension is in our video.
Another limiting factor may be the proximity of the rear boundary or a highway near your rear boundaries. In certain circumstances, you will need to apply for planning permission.
A further limiting factor is the impact of the depth on the type of roof you can have. A 3m depth will allow a lean-to roof as the pitch angle is sufficient to use roof tiles or slates. As the depth increases and the pitch flattens, you have to consider using either a flat or double-pitched roof.
The usual caveats apply to buildings that are listed or located in a designated area.