use an Architect - get the design you want, and specification sorted before approaching any builders. In this way you:
- develop the design you want using a professional team, exploring layout options in detail. with any extension it is important to get the whole house balance/accommodation right to maximise value.
- develop structural designs to work best with layout
- understand party wall matters and approach correctly, this can be a major cause of disruption (ie cost and delay)
- get a good understanding of costs through design process
- go to a number of builders with a defined set of drawings/scope and get good comparison costs
- pick the builder that works best for you in terms of quality, cost and time.
- get the right contract
- understand your rights as the building owner and get the right contract.
following this process puts you in control, and mitigates problems on site, and therefore cost escalation. using an architect saves you cost and time - and improves quality.