It is common policy amongst nationwide planners that an extension roof is subservient to (lower) than the original. No designer would deliberately draw it otherwise because he knows it would be knocked back, in about 90% of cases.
It doesn't sound like the roof designer considered the planning issue, but given they advised to check with the planning officer, I doubt a claim exists. At the end of the day you ran in to a change control issue with the existing roof and the builder remedied it in a way that you agreed to. At best you might be able to get mates rates for any remedial work.
Wait for the Council to issue an enforcement notice, if they do put, in a written reps appeal ideally with help of a planning consultant. It will be less, maybe much less, than the cost of altering the roof line but you can leave that decision until later.
If the Council doesn't enforce, wait 4 years and apply for an LDC.
I take the point about mates rates but when has a builder ever been anyones mate?
What exactly does your letter say and what has happened since as what you've said so far doesn't follow planning policy.
With a planning breach the following is usually what happens,
Council contact you to say they've been informed of a planning breach
Planning officer will visit site to confirm the breach, if your there they'll normally engage with you to understand why the breach occurred.
You'll receive further communication confirming if there has been a breach, you will then be invited to either submit a retrospective planning application or a new planning application which is basically a revised set of drawings including the breached part.
Council will then either pass or refuse your application, if you refuse to submit an application they will hold a meeting to decide if the breach is acceptable or not based on there own gathered evidence.
In the event of refusal you'll be given the right to appeal, once this expires or the appeal is refused the council will decide if they will issue an enforcement notice requiring you to alter your building.
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