In this case, you would be the property developer.
The developer uses a builder (or whatever means is necessary - architects, engineers, QSs, direct labour, etc.) to develop the properties for greater value. The builder comes in where building is needed to provide the changes required. So the names are the key.
I'm not aware of any developers who offer a service where they assist in the planning of developments, but it's something I've never been involved in so may well be available with some searching.
As for demand etc., very tough for anyone to give a great answer with the current market. An estate agent (EA) in the local area might be one possible route to investigate for assessing the most prudent options. Talking to a few of the builders might also give some beneficial advice while you get a chance to size them up for the potential work. Other avenues would include getting a good architect on board, these are the real people with a flair for design to provide ideas, however I'm not sure how financial/economic focused any individual architect might be (they're trained to design what's striking, not what's profitable).