Side Extension London 2026: Complete Guide to Costs, Design and Planning

A side extension makes use of the plot alongside your property to add substantial ground floor or two-storey space. It works particularly well for semi-detached and end-of-terrace houses with side access, and for Victorian and Edwardian terraces with side return alleyways. This guide sets out the different types of side extensions, current 2026 costs, planning permission rules and design considerations across London and Hertfordshire. LCCL Construction has delivered hundreds of side extensions across London as part of our complete design and build service.

LCCL Construction — specialist side extension builders across London

Types of Side Extension in London

Three main types of side extension in London. First, side return extensions— fill the narrow alleyway alongside a Victorian or Edwardian terrace back addition, typically used to expand the kitchen. Second, single storey side extensions on semi-detached houses — build outward on the side plot creating a substantial ground floor room (utility, playroom, home office, extended kitchen). Third, double storey side extensions on semi-detached houses — add bedrooms and bathrooms upstairs plus a substantial ground floor room downstairs. Each type suits different property styles and delivers different amounts of space.

Side Extension Costs in London 2026

Side return (terraced): £42,000 – £93,600 inc. VAT (£35,000 – £78,000 + VAT).

Small single storey side extension (semi-detached): £42,000 – £90,000 inc. VAT (£35,000 – £75,000 + VAT).

Standard single storey side extension: £57,600 – £74,400 inc. VAT (£48,000 – £62,000 + VAT).

Double storey side extension: £90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT).

All include full design and build. The final figure depends on postcode, access and site conditions — the instant estimate applies your area.

Planning Permission for Side Extensions

Permitted development rights for side extensions are more restrictive than for rear extensions. Under PD a side extension must be single storey with maximum eaves height 3 metres, and its width must not exceed half the width of the original house. Any two-storey side extension requires full planning permission. Side returns on terraced houses often exceed PD limits or fall in conservation areas or Article 4 direction zones. LCCL Construction handles all planning applications. See our full house extension planning permission guide.

Best Design Approaches for Side Extensions

Successful side extensions typically use one of three design approaches. Contemporary — full-height glazing, contrasting cladding materials (dark timber, corten steel, standing seam zinc), stark modern lines. Best for family homes where the extension is intended to feel distinctly new. Traditional — matching brickwork, matching roof tiles and pitch, sash windows aligned with existing elevations. Best for conservation areas and period properties. Hybrid — matching materials at ground floor with contemporary elements above (contrasting first floor cladding, oversized windows, contemporary roof profile). Best for semi-detached properties in outer London where planners often prefer subordinate design.

What Is Included in a Side Extension Cost?

LCCL Construction side extension costs include: measured building survey, architectural design, planning permission application, building regulations, structural engineer calculations, party wall notices where required, groundworks and foundations, superstructure with matching brickwork, roof structure and roofing, windows and doors including any bifolds or sliding doors, structural steelwork to break through to the existing house, first fix electrics and plumbing, insulation, plastering, second fix and decoration. Kitchen fittings and specialist finishes are typically client-supplied.

How Much Value Does a Side Extension Add?

On our projects and local sold prices, a well-planned project of this kind usually adds more to the value of the home than it costs to build — see our guide on how much value an extension adds. A single storey side extension costing £65,000 typically adds £115,000-£180,000. A double storey side extension costing £100,000 typically adds £175,000-£275,000. See our full guide on how much value an extension adds and home extension cost guide.

Can it be done under permitted development?

A single-storey rear extension of 3 m on a terrace or semi (4 m on a detached house), a side-return infill and a side extension up to half the original width are permitted development on a house outside a conservation area or Article 4 street — no planning application, just a Lawful Development Certificate, which we apply for on every one. Fixed-price permitted development extensions start from £42,000 inc. VAT (£35,000 + VAT) at economy specification, design and build inclusive, and the 6 m / 8 m larger home extension goes through a 42-day prior approval notification rather than a full application. See our permitted development projects page for every product with its from-price and the checks we run before we confirm a project qualifies. Not sure whether your project needs a planning application? Check if you need planning permission

Frequently Asked Questions About Side Extensions in London

How much does a side extension cost in London?

From £42,000 inc. VAT (£35,000 + VAT) for a small side return to £198,000 inc. VAT (£165,000 + VAT) for a large double-storey side extension.

Do I need planning permission for a side extension?

Two-storey side extensions always require planning. Single storey side extensions can fall within permitted development if width does not exceed half the original house width, but most London side extensions require planning.

Can I have a side extension on a terraced house?

On terraced houses this typically takes the form of a side return extension filling the alleyway. True side extensions (building outward on the side) are only possible for end-of-terrace properties.

To book a free consultation call LCCL Construction on 020 8037 7332 or get an instant quote online.