Dear @PresentingPercy, @Seeline, @tilder,
Apologies for the delay in answering - life gets in the way...
Here's the statement in the rejection letter
The proposal by virtue of its design, scale and siting is considered to result in a dominant and visually harmful development that fails to enhance or enrich the appearance of the existing dwelling or the streetscene. By virtue of its scale, design and location the proposal is considered to result in overdevelopment of the site and a form of development which would be substantially harmful to the existing dwelling and streetscene
I don't want to post here the sections of the local plan which have been highlighted as that's a bit outing, but I'd be happy to share with anyone by PM if you're still interested/curious. It's not that clear to me exactly which sentence in the section they've highlighted is the issue but I guess it's enhance ... the settlement edge, space between settlements, and their wider landscape setting. I'd hoped that the rejection letter would have some detail in it - the section highlighted encourages areas that maximise the opportunities for public art. We didn't do that either....
@PresentingPercy - those are rather beautiful rooms over garages, and our plan is nowhere near as beautiful or as big (again happy to share via PM) It does square off the house (but fills in the space which is the no-no) and covers a rather unattractive flat roof. We get caught in the mixed messages of an extension needing to be different to the main building and at the same time unifying it. We have tried both approaches in the two plans we submitted.
@Seeline the planning officer wouldn't accept anything in the space, even bringing it in from the boundary. In in the second plans we pushed it back a bit to try and make it subservient. And thank you for your advice re the accreditation of the planning consultant - this is all new to me to all advice is gratefully received.