https://www.planningportal.co.uk/permission/common-projects/extensions/planning-permission

Thanks for this Sarah, all it ever seems to say though is ‘Single-storey rear extensions cannot extend beyond the rear wall of the original house* by more than four metres if a detached house; or more than three metres for any other house.’

But what I can’t tell is whether it is each rear wall or the rear most wall…

It's measured from each part of the original wall - so your second option is the correct one. Unless the bit next to the utility was done before 1948?

Here’s the technical guidance. P19 covers your question with some example plans.
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/830643/190910_Tech_Guide_for_publishing.pdf

Perfect, thanks all!

Planning Portal states you can extend up to 6m, or 8m if detached, within permitted development rights.

Ignore me, don't know where I read that

@Sarah L

You’re right you can extend 6/8m but you need Prior Approval from the LPA and they consult with the neighbours

https://www.planningportal.co.uk/permission/common-projects/extensions/planning-permission

Is there a reason you're looking to restrict yourselves to be within permitted development? It's not that difficult to get planning permisison if what you want isn't strictly within the limitations of permitted development.

You'll find the videos I've made about planning permisison on my youtube channel useful: https://www.youtube.com/@iarchitect

Best wishes with your home redesign!

Jane