Extension Budget Spreadsheet for London — Loft & Extension Planner

How to use it

1. Replace every placeholder with figures from your drawings and tenders.
2. Keep provisional sums visible — do not hide them inside a single “build” number.
3. Fill works bands (structure, services, finishes), then prelims and access.
4. Add professional fees as real appointments, not guesses left blank forever.
5. Set a contingency % you can defend, then convert it to pounds against the works subtotal.
6. Optional: add a client-side PM line — see project manager cost. Professional fees row context: architect fees UK & London.
7. Revisit after each tender round.

What is on the sheet

• Project meta (borough, house type, live-in, scope tags)
• Works — structure, services, finishes
• Prelims and access
• Professional fees
• Contingency % → £
• Optional client-side PM
• Working total + open risks notes

Already holding a builder’s price? Run it through the extension cost overrun calculator first — it works out what that quote becomes once missing scope, variations and programme slip are in. Fees: how PM fees work.

Pair with the builder quote comparison checklist and the 2026 extension & loft costs guide. Decision reads: when a PM makes sense · PM vs architect CA.

No quotes yet? Get a sourced London starting range from the extension cost calculator or the loft conversion cost calculator, then itemise every professional and statutory fee with the renovation fees calculator — and carry the figures into this planner.