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Garage, 4 bedroom and ensuite extension

Hi, I'm wanting some advice and guidance on extending out 91m2 cottage. I've been drawing up some designs on how i'd like it layed out. But i'd really like to have someone come and have a look at the site and see what could be done and to answer some questions. Do i need an architect for an extension? If I got a draftsman to do up the plan, does the builder organize consents? Earthworks if needed? What's a good/standard price for additions? Thanks