Our house (in the middle of...)...has a restrictive covenant that the previous owners broke by building an extension.
Thought that might have been flagged during planning but nope...
Our solicitor got them to take out an insurance policy that covers anything should the council say "hold on a minute you crafty buggers".
I think the council have a time limit of 20 years to raise an objection to it. Half way through that.
A covenant isn't a material considerations when a Council considers and determines a planning application, so there's no fault of the Council there (for a change). They have no ability to enforce against any breach of a covenant, only whether the extension is lawful in planning terms, so that could well have been a pointlessly expensive policy!
The Council's time limit for enforcement is ten years - but, as above, all they can consider is whether the extension is lawful in planning terms (sowhether it's been built without consent, is different to what was granted consent, or is in outside the scope of the dwelling's permitted development rights).
If your extension was built in accordance with a planning permission the Council granted, or was in accordance with the legislation which essentially grants you permission to build the extension subject to it confirming with X, Y and Z, you're not going to have the Council knocking on your door.