Start with the brief
Before drawings, write down what problems the extension needs to solve and how you want the space to feel. This brief becomes the yardstick for every later decision.
Feasibility and budget
Get a realistic cost per square metre for your area, add 10–15% contingency, and check the plot against planning constraints before spending on detailed design.
Design, permissions and tender
Work through concept, planning, technical design and tender in that order. Skipping stages is the most common cause of overspend and delay.