You are right that making a start to the works effectively keeps your old permission alive indefinitely. However, this only applies if you have cleared any conditions attached to the permission that need the approval of the council before you start any work. If you go down this route make sure you inform the planners, in writing, that you’ve made a start and always keep documentary evidence of when the work was done.

Alternatively, there is a new simplified renewal mechanism, whereby you can apply to extend the time limit of your permission by a further three years. It’s a lot cheaper and easier than submitting a new full application and should usually be approved automatically. The only reason the application can be rejected is if something significant has changed since the last permission was granted.