Topic 05

House extensions

Planning, design and technical guidance for rear, side-return, wraparound and two-storey extensions.

27 articles

Choosing your extension

01Which extension suits my house: terrace, semi or detached?The suitable extension depends on your existing layout, available land, neighbours and planning constraints—not just whether the house is terraced, semi-detached or detached. Compare how each option improves the whole home before choosing its size.02Rear, side-return, wraparound or two-storey: the options comparedA rear extension adds space behind the house; a side-return extension fills space beside a rear wing; a wraparound combines side and rear work; and a two-storey extension adds accommodation on two levels. The right option depends on the layout gained and the permissions and structural work required.03Extending a Victorian terrace, an Edwardian house or a 1930s semiA period house should be extended around its actual structure and architectural character, rather than a standard “Victorian”, “Edwardian” or “1930s” template. Establish the original layout, later alterations and any heritage controls before choosing the form of the addition.04Will an extension add value to my home?An extension may increase your home’s market value, but there is no reliable universal percentage and the increase may be less than the complete project cost. Assess the proposed finished home against relevant local sales, separately from the value of the extra space to your household.

Permissions

05Do I need planning permission for a rear extension?A rear extension may not need a householder planning application if it meets every applicable permitted development condition. Larger single-storey proposals may instead need prior approval, while proposals outside those rights need planning permission from the council.06How far can I extend without planning permission?For an eligible house, the ordinary single-storey rear permitted development limit is 3m for an attached house and 4m for a detached house. The larger 6m/8m route requires prior approval, and all routes remain subject to other restrictions.076m and 8m rear extensions: the larger home extension route step by stepAn eligible larger single-storey rear extension can reach up to 6m for an attached house or 8m for a detached house, but the council’s prior-approval procedure must be completed before work starts. The dimensions are maximums subject to the other permitted development conditions, not guaranteed approvals.08Side and side-return extensions: what's allowedA side or side-return extension can be permitted development only where it meets the side-extension rules and any rear-extension rules that also apply. Infilling a narrow gap does not automatically qualify, especially beside an original rear wing or in a conservation area.09Wraparound extensions: why they usually need planning permissionWraparound extensions often need a planning application because the combined side-and-rear enlargement fails one or more permitted development limits. Some designs can qualify, but the complete shape must be tested against both the side and rear rules.10Two-storey and first-floor extensions: the 45-degree rule and neighbours' lightThe 45-degree test is a design assessment used in some council guidance, not a single national rule that guarantees permission. Two-storey and first-floor extensions must satisfy the applicable planning route and the council’s assessment of light, outlook, privacy and design.11How close to the boundary can I build, and how high?There is no single planning rule requiring every extension to stand a fixed distance from the boundary. For Class A permitted development, an extension within 2m of a boundary has a 3m eaves limit, but other height limits, building regulations and property rights still apply.12My council's extension design guide: what it saysYour council’s current extension design guide explains how it assesses matters such as scale, roof form, neighbouring windows and the character of the house. Read it alongside the applicable development plan and constraints; it is not a substitute for the national permitted development rules.

Design

13Flat roof or pitched roof?A flat or pitched roof can be suitable for an extension; the choice depends on the permitted height, relationship to the house, drainage and the roof’s technical build-up. Compare the complete roof section, including insulation and structure, rather than its external appearance alone.14Rooflights, bi-folds and large glazing: how much glass is allowed?There is no absolute 25% ban on glazing in an extension. Approved Document L’s standard approach limits the combined area of windows, roof windows, rooflights and doors to 25% of the extension floor area plus qualifying openings lost from the existing house, but alternative calculated compliance routes are available.15Open-plan kitchen extensionsAn open-plan kitchen extension is possible, but the design must address structural support, the escape route, ventilation and energy performance together. Removing the back wall and the door to the stairs is not just a kitchen-layout decision.16Building over or next to a garageBuilding over or beside a garage may be feasible, but the garage’s walls and foundations must be assessed and the planning route checked separately. An existing single-storey garage is not evidence that another storey is structurally suitable or automatically permitted.17Building up to the boundary or raising the party wallYou may be able to build up to a boundary or raise an existing party wall, but the two situations follow different legal rules. A new wall astride the boundary needs the neighbour’s written consent; work to an existing party wall must follow the applicable Party Wall Act procedure.18Replacing a conservatory with a proper extensionReplacing a conservatory with a normally heated extension usually requires building regulations approval, and the planning position must be checked afresh. Keeping the same footprint does not prove that the replacement is permitted development or that the old foundations can be reused.19A joint extension with your neighbourYou and your neighbour can coordinate extension designs and construction, but each property’s planning position, ownership rights and responsibilities still need to be resolved. Building together does not create a shared permitted development allowance or remove the need for party wall procedures.

Technical

20Building regulations for an extension: what's checkedBuilding regulations for an extension cover the safety and performance of the new work and relevant alterations to the existing house. The design normally needs to address structure, fire safety, moisture, ventilation, energy efficiency, drainage and the services being installed or changed.21Foundations for an extension: depth, trees and clay soilThere is no standard foundation depth that is safe for every London extension. The design depends on ground conditions, the building’s loads, trees, drains and nearby structures, with investigations and the excavation inspection informing the final solution.22A drain or manhole where the extension goesA drain or manhole in the proposed footprint does not automatically prevent an extension, but its ownership, route and level must be established before the design is fixed. Public sewers and lateral drains can require Thames Water’s agreement, and its specification does not allow inspection chambers inside the proposed building.23Party wall notices for an extensionAn extension can require party wall notices for work to an existing shared wall, a new wall at the boundary or qualifying excavation near a neighbouring structure. Whether a notice is needed depends on the actual work, not simply whether the extension has planning permission.24The steel beam for opening up the back of the houseOpening up the back of a house normally needs an engineer to design how the loads formerly carried by the wall will be supported. The solution may involve a steel beam, columns and foundation work; the beam cannot safely be selected from the opening width alone.

Costs and timelines

25How much does an extension cost in London: rear, side, wraparound, two-storey?The cost of a London extension depends on the complete scope, existing structure, ground conditions and specification; its description as rear, side, wraparound or two-storey is not enough to price it. Use coordinated drawings and itemised quotations, including fees, VAT and work to the existing house, rather than an unsupported price per square metre.26How long does an extension take, from first drawings to completion?The full extension programme includes design, approvals, technical preparation, construction and completion—not just the weeks when builders are on site. There is no dependable universal duration: ask for a project-specific programme showing decisions, permissions, investigations, ordering and inspection stages.27Can we live at home during the extension?You may be able to remain at home during an extension, but only if the occupied part can be kept safe and usable throughout the relevant stages. Agree the separation, access, essential services and temporary move-out arrangements with the contractor before deciding that staying is practical.