ignoring the ettiquete of the site, even if we were allowed to discuss cost, it is a very variable thing, from hundreds to thousands of pounds depending on what is already there, the geometry of the building and after that local labour rates and so on are not fixed. Use the yellow pages, and make few calls tonight, and get a few to walk through it - this will be much more precise.
If you're just having a shell built and managing the fit-out yourself then you are in for a long ride. It can be a rewarding experience when you're finished but you're sure to gain a few grey hairs in the process. 3 rooms sounds like a fairly sizeable extension too.
This is a great place to ask specific questions but general ones like yours are better answered by studying texts on the subject long and hard. Certainly project managing is something that needs lots of research.
You could do a lost worse than get a subscription/back-issues to Homebuilding and Renovation. They have a series running on working with each of the trades. In June it was plumbers and I have a sneaky feeling this month is Electricians but I can't find my copy at the mo. They show typical costs applied at a whole house level but you can pro-rota.
At the end of the day though it's not like you're just getting some rough ideas down to see if the project is viable. You're going for it so ultimately the only satisfactory thing to do is to get some real estimates now.
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