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Extensions 10 min read

Planning Your House Extension

From first sketch to shovel-ready drawings, learn the practical steps that lead to a smooth, well-priced extension project.

Last reviewed: July 2026

Reviewed by BuildGuide Editorial Team

Full review date: 1 July 2026

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.

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