Bespoke House Extensions in Cardiff and the Vale 

A standard single-storey extension typically ranges from £1,800 to £2,500+ per square metre for general construction, while high-spec finishes, complex knock-throughs, or bespoke architectural designs commonly exceed £2,500 to £3,200+ per square metre. For a standard 20m2 kitchen-diner extension, budgets generally range between £40,000 and £65,000+ before VAT and internal fittings (such as premium kitchens or bi-fold doors). Double-storey extensions usually cost around 50–60% more than a single-storey equivalent, offering a lower cost per square metre.

Under standard Permitted Development (PD) rights, the footprint of any proposed extension, combined with all existing extensions, sheds, outbuildings, and detached garages, must not exceed 50% of the total land area around the "original house" (the property as it stood on 1 July 1948 or when first built). The footprint of the original house itself is excluded from this 50% calculation.

The major cost drivers fall into two categories:Groundworks & Structural Alterations: Laying deep foundations (especially near tree roots, poor soil, or public sewers) and installing extensive structural steelwork (RSJs) to knock through existing load-bearing walls for open-plan living. Fit-Out & Glazing: High-end finishes drive overall costs up rapidly, specifically bespoke designer kitchens, luxury bathrooms, underfloor heating systems, and large-format structural glazing or premium aluminium sliding/bi-fold doors.

Under Permitted Development rules (assuming standard rights apply and the property is not listed, in a conservation area, or restricted by an Article 4 direction):Single-Storey Rear Extensions:Can extend up to 4 metres beyond the original rear wall for a detached home, or 3 metres for a semi-detached or terraced home (in Wales, standard PD allows up to 4 metres subject to local conditions).Maximum total height is 4 metres. If within 2 metres of a boundary, the maximum eaves height cannot exceed 3 metres. Single-Storey Side Extensions:Cannot exceed half the width of the original house and must be single-storey with a maximum height of 4 metres. Multi-Storey Extensions:Cannot extend more than 3 metres beyond the original rear wall and must sit at least 7 metres away from the rear property boundary.