What limits apply under Permitted Development for the addition of a single-storey rear extension to a semi?

Its best to look at the guidance provided by the Planning Portal for the complete list of conditions applicable to under Permitted Development rights. The main limits are a maximum depth of 3m, extended to 4m via prior approval; maximum 4m to ridge height and 2.4m to soffit from ground level, with a further restriction on height to the eaves of 3m if the extension is within 2m of a boundary; total area of all extensions must be less than 50% of the curtilage area.

Its always a good idea to apply for a Certificate of Lawful Development; while not mandatory, it provides peace of mind and there is a growing tendency for such documentation to be requested by conveyancing solicitors in a property sale.