Neighour wants conservatory extension

Your proposal sounds like the best of both worlds but I fear it may become a potential issue in the future especially if one party wants to move or if there are maintenance issues. Firstly you would need to check with planning dept such a development would be acceptable. Secondly the design would have to be approved by building control. As such your first port if call should be an architectural technician or higher. It's an interesting concept though ...

I'd like to add to JWP Builders reply in that the point of a conservatory is to let light in, if the roof is a terrace or balcony this would not be the case and it wouldn't be a conservatory.

first point of call is to speak to the freeholder and check the party wall act isnt applicable then enter an agreement with the freeholder in case ownership of leases change take advice on whether it requires planning or is it permitted development write informally to the planners ive always rung them in the first instance at that point pay for your plans and drawings and make sure your roof is sound insulated good luck they look great

Party wall agreement is very important, I agree everyone better to build in his property land thanks fatjon renovations ltd