Extension costs vary enormously depending on specification, location and complexity — but homeowners deserve realistic figures before they commit to design fees. Here is an honest breakdown based on our recent projects across Essex and the South East.
A single-storey rear extension of 20–30m² typically costs between £55,000 and £85,000 including VAT, professional fees and landscaping reinstatement. This assumes a mid-to-high specification: bi-fold or sliding doors, underfloor heating, a well-appointed kitchen and quality finishes throughout.
Double-storey extensions offer significantly better value per square metre. A 40–60m² double-storey rear extension — providing a kitchen-diner below and bedroom or bathroom above — typically falls between £90,000 and £160,000. The groundworks, foundations and roof structure are shared across both floors, reducing the cost per square metre by 25–35% compared to two separate single-storey builds.
Side-return extensions in Victorian and Edwardian terraced houses are a popular option in East London and inner Essex. These smaller projects (typically 8–15m²) cost between £40,000 and £65,000 and can transform a narrow galley kitchen into a sociable open-plan space.
What drives the cost up? Structural complexity (removing load-bearing walls, steelwork), high-end fixtures (Bulthaup or SieMatic kitchens, Italian tiling), and challenging site access all increase the budget. Conversely, a simpler specification with standard fittings and straightforward foundations will sit at the lower end of our ranges.
Every project we quote is fixed-price. We provide a detailed, itemised specification so you know exactly what is included — and more importantly, what is not. No surprises, no provisional sums, no hidden costs.