This is something that comes up again and again and again.
Following a lengthy design process and planning permission is obtained or an Exempted Development design is proposed. The client makes changes to the design and doesn’t obtain a new planning permission for the changes.
THIS ADVICE NOTE covers my standard procedure for handling this situation if the changes are minor. If ‘in my opinion’ the changes are more than minor then a new planning permission will need to be obtained.
This advice note (as well as a heap of others) are included in the right hand widget side-bar.