Re: How much are builders and architechts charging
The renovation and extension would take the best part of a year to complete. Planning Permission (all going well) takes about 8 weeks from date of application, depending on your local authority. Design and drawings would add another few months (or many months, depending on whether you know what you want, how long it takes you to make decisions, your architects skill at keeping a project moving etc etc etc.). And don't forget it'll take another few months to close the house purchase even if your offer is accepted tomorrow.
Build costs? Again, this varies around the country and there are different numbers for renovation and new build. Sounds a little high, but he may be erring on the high side because he knows all projects have a tendency for "scope creep". Do you have a good reason to doubt your architect's numbers? Choose your architect carefully, you need to understand each other and you have to be confident that the architect has your interests at heart. S/he acts as your agent throughout the process and a good architect more than earns the fee charged. Did the architect talk numbers with you and give you a sample contract and explain the process?
Don't worry about the EA, probably pulling your leg, but there's plenty of houses out there for sale if you don't get this one.