Forget asking your planning department for advice (apart from asking where to find the right document), they won't give any advice out except to read the local plan, you should be able to find your Local Plan Guidance on your local authority's website, that's the document that will dictate the design constraints the planners will use to determine your planning application, for example they may say you must not build up to the boundary or that the front portion must be set back by say 1m, every council has a different local plan, no two councils have the same requirements, so that's where you should start.
Sometimes it can be difficult to find on local authority websites but start by looking at you LA's planning website and see if you can find it or google 'XXXXXX (local authority) Local Plan' or similar it should be a big PDF or maybe give them a ring to ask them to point you in the right direction, if no luck then let us know your local authority. Then search for the word 'extension' and you should find the right section. For example this is the Local Plan for where I live:
https://www.teignbridge.gov.uk/media/1669/local-plan-2013-33.pdf page 56 deals with extensions to dwellings, our one is very vague but yours will be different so you need to find it. Then you will see what, if any restrictions they apply when deciding it. Then go from there.