Immunity for PD after a certain time?

Browsing threads on here yesterday, I came across the bombshell that most bungalows can't achieve a rear extension under Permitted Development for one reason or another, mainly eaves heights dictate low headroom inside, or as in the attached diagram, you CAN NOT do as the Planning Portal say you can because that little triangle of the roof over the existing build alters the existing roof, not allowed under PD.

I'm sure I'm not the only one in the country who has been tripped up by this. I know of at least 3 in my street who have some a 4m length single storey extension as the diagram under PD. So, by law, an enforcement officer could have them removed?

I also saw on a thread that these situations become immune from enforcement after 4 years, but I have no idea if this is true or not.

Any solid information or experience of this daft rule?

The statutory time limit for enforcement action on unlawful development is 4 years.

Many people build things without planning permission and get away with it. The council doesn't usually get involved unless a neighbour makes a complaint. If the breach of the rules is not too great, they may just invite the owner to make a retrospective planning application, but there's no legal requirement for the owner to do that.

The eaves height needn't necessarily dictate the internal headroom. Raised tie or vaulted roof would sort that out.
The problem with bungalows I have come across is a wide extension with a single ridge roof results in a ridge too high and would look ridiculous.
I designed this one a few years back. The clients absolutely love it inside. Light, airy, interesting and a talking point apparently.

Eaves height specifically in flat roof scenarios. In a flat roof scenario, the eaves are measured from the adjoining ground, vertically to where it meets the top surface of the flat roof. If this (eaves), can be no higher than say a typical 2.1m existing eaves, to meet PD, the inside space will be very low