Estimating any building project is not an exact science as each builder has their own pricing strategies and margin allowance which can lead to great variations in their quotations.  This makes estimating budget costs prior to developing the full scheme design very difficult and any estimated costs offered by Steven, particularly at feasibility stage, should only be treated as very approximate and not as the maximum price expected which can only be determined by a tender.

However, from previous experience there are a few common items that can affect your extension building cost:-

1. The cost per square metre is significantly higher for smaller extensions below 30 square metres.
2. Complex schemes and works that affect the existing structure will increase the total building costs of the extension.
3. Bad ground conditions, trees and hedgerows etc. can lead to deep foundations and a specially designed ground floor structure.

It has been my experience that current building costs for extensions generally range from £1,200 to £1,500 per square metre plus VAT and other lump sums for appliances and fittings.  

Builders are very busy, you may experience difficulty in obtaining a builder that can start your project shortly after Building Regulation Approval.

When calculating the area of the extension for budgeting purposes it is wise to use external measurements for each floor - not just the footprint area created.