The Government have just announced that the May 2019 date when “larger extensions” under Class A Permitted Development Rights have to be built by has been dropped and the change made permanent.
This means that detached dwellings can have single storey rear extensions up to 8m deep and semis / terraces up to 6m deep…!
It’s not quite that straightforward (whatever is?) and you should always take professional advice as there are lots of tricky little ways to trip up with PD Rights and it doesn’t apply everywhere (e.g. Conservation Areas, National Parks, Listed Buildings etc.) and if you’ve already had extensions carried out they may preclude. Likewise, conditions attached to any existing planning permission may have already withdrawn Class A PD Rights…
Using the “larger extensions” PD still requires prior notification to the local planning authority who will then consult your neighbours as well; if there are no objections then the LPA should approve the application by default.
The process is the same that it has been since they brought in the “larger extensions” a few years ago; the change is that you no longer MUST complete the build by May 2019.
Remember, if in doubt, take professional advice!