Mistake 1 — Skipping the preliminary diagnosis: a wall that looks sound can hide damp, asbestos or a structural problem. The diagnosis costs €500 to €1,500 but can save you €30,000. Mistake 2 — Comparing quotes on the total price only: two €50,000 quotes can hide very different realities. Analyse them line by line. Mistake 3 — Not checking insurance: ten-year structural warranty, professional liability — demand valid, current certificates. Mistake 4 — Changing the project mid-works: every change generates an average cost overrun of 15 to 20%. Freeze your project before starting. Mistake 5 — Paying too early: stick to the contractual payment schedule. Never pay more than the actual percentage of completion. Mistake 6 — Failing to formalise in writing: every verbal agreement must be confirmed by email or letter. Mistake 7 — Neglecting the works handover: this is the moment to record every reservation. After handover, your negotiating power drops drastically.