Permitted development advice please

[wessex101]

I'll have another go. I have done a simple sketch to illustrate the potential stumbling block.
The planners could take the wall of the small projection highlighted in yellow as a "rear elevation" therefore the proposed extension will be more than 4m. Now this is a rather tenuous interpretation but the planners can be pedantic little sods.
Personally, if everything else is within PD I would just build it and in the unlikely event it gets picked up just plead ignorance as it is so close to being permitted development it is an easy one to miss. Then if the planners get stroppy apply for planning retrospectively and it would probably be approved. It is risky though.
Alternatively if you are risk averse apply for a lawful development certificate. Technically you shouldn't get it but I have seen plenty of similar extensions that have, even though they were not strictly PD. Option 3, bite the bullet and submit a planning application.

 

[DevilDamo]

If the existing layout is the ‘original’ layout minus the conservatory, Alternative 1 should be considered PD but subject to the Prior Approval scheme. I can’t see how Alternative 2 would be considered PD as the part on the right technically has a front wall. If this part was not original, then it could be considered PD but again subject to the Prior Approval scheme.