Rear extension architects in London, adding space and light without moving.

A rear extension architect is a specialist who designs the ground-floor space London homes are missing — the open-plan kitchen-diner, the family room that reaches the garden, the side return that turns a dark galley kitchen into the best room in the house. It begins with a brief — a Victorian terrace in Harrow with a kitchen nobody uses, a 1930s semi in Pinner where the family has outgrown the downstairs, an Edwardian house in Stanmore cut off from its garden — and ends with a complete set of drawings and specifications a builder can work from without guessing.

There's a common misconception that a rear extension is just "a box on the back." It isn't. In fact, the difference between a good extension and a bad one is rarely the size. It is the light, the ceiling heights, the width of the opening, where the steel goes, and how the new space meets the old. Get those right and the whole house feels bigger. Get them wrong and you have spent six figures on a dark corridor with bifolds.

In practice, rear extension work at Extendopedia covers everything from the first site visit and feasibility sketch through to concept design, planning applications or permitted development, building regulations drawings, structural coordination, party wall notices, tendering, and support during construction.

Decisions made in the first few weeks of an extension project stick around for decades. On a competitive London street, that matters — a well-planned extension adds more than its cost at resale, while a poorly planned one is worth less than the money spent building it.

• Daylight. The number one complaint about rear extensions is darkness at the back of the plan. Roof lights, the width and height of the rear opening, and the depth of the extension all decide how far daylight reaches. We model this before you commit, not after.
• Flow. A kitchen-diner only works if the route from front door to garden makes sense — cooking, eating, sitting and playing zones in the right order, without circulation eating the floor area you paid for.
• Proportion. Ceiling height, opening widths and the junction between old and new decide whether the extension feels generous or mean. A 2.4m ceiling under a flat roof feels very different from 2.7m with a roof light over the dining table.
• Structure. The steel beam that makes open-plan possible has to go somewhere — its depth affects ceiling heights and door head heights throughout. Structural thinking belongs in the concept design, not as an afterthought.
• Value. Estate agents typically credit a well-designed rear extension with adding 10–20% to a London home's value — but only when the design is good. A dark, awkward add-on adds cost without adding value.

Every extension runs slightly differently, but a well-run project follows the same broad shape. Here's what happens at each stage.

1. i.

   Brief & feasibility

   A conversation about how you want to live, followed by a site visit where we measure the house and garden, check the light, and confirm what the site and budget can realistically deliver. We give an honest read on feasibility — including when the answer is "extend less, reconfigure more."

2. ii.

   Concept design

   Two or three layout options for the new ground floor: where the kitchen sits, how wide the opening goes, flat roof versus pitched, how the garden is reached. You see scaled drawings — not just words — and choose the direction to take forward.

3. iii.

   Planning route

   We confirm whether the scheme can proceed under permitted development or needs a householder application, and prepare whichever route applies. See section 06 for the rules that decide this.

4. iv.

   Technical design

   The chosen concept becomes buildable: construction drawings, structural coordination with the engineer, drainage, insulation and ventilation strategy, electrical and lighting layouts. This is the information your builder prices from.

5. v.

   Building regulations & party wall

   Full plans submitted for building control approval; party wall notices served on neighbours where the work affects shared walls or excavates near boundaries. Both run in parallel with tendering.

6. vi.

   Tender & contractor

   We help you get comparable, like-for-like prices from reputable builders — and read the quotes with you, because the cheapest price is rarely the cheapest project.

7. vii.

   Construction support

   We answer the builder's questions on site, review key stages, and check that the built work matches the design intent. This is where Principal Designer duties sit under the Building Regulations regime.

Strip away the trends and most respected extensions share the same handful of fundamentals. These are the six principles we hold to on every project at Extendopedia.

Most London rear extensions fall into one of five categories. Knowing which applies to your house helps you budget, plan permissions, and shortlist the right team.

• Side return infill. On Victorian and Edwardian terraces, the narrow alley beside the kitchen is roofed over — typically gaining 2–3m of width along the full depth of the house. The highest value per pound in London: modest cost, transformative effect on the kitchen.
• Full-width rear extension. The back wall comes out across the full width of the house, usually 3–4m deep. Creates a proper open-plan kitchen-diner. The workhorse of London extensions.
• Wraparound (L-shaped). Side return plus full-width rear combined — the maximum ground-floor gain on a typical terrace. Bigger budget, bigger structural moves, biggest transformation.
• Double-storey rear. Extends both ground and first floor — a new kitchen below, a new bedroom or bathroom above. More complex structurally and almost always needs full planning permission, but doubles the added space.
• Glass box / orangery style. A highly glazed extension — frameless corners, slim-profile sliding doors, roof lanterns. Stunning when done well; demands careful thermal design so it works in January as well as July.

Three separate systems, all of which apply to most rear extensions. Here is how each one touches your project.

• Planning permission. Many single-storey rear extensions fall under permitted development: up to 3m deep on a terraced house or 4m on a semi-detached or detached house under standard rules — or up to 6m/8m respectively under the larger home extension scheme, which requires neighbour consultation. Other limits apply: maximum 4m height, eaves no higher than 3m within 2m of a boundary, no more than half the garden covered, materials similar in appearance. You'll need a full householder application in conservation areas with Article 4 directions (several parts of Harrow), for double-storey extensions, or anything beyond the limits. Either way, we recommend a Lawful Development Certificate on every permitted development project.
• Building regulations. Separate from planning, and always required. Covers structure (foundations, steelwork, lintels), thermal performance, drainage, ventilation, electrics and glazing. Our drawings are produced to satisfy Building Control first time — approval before the builder starts avoids expensive mid-build changes.
• Party walls. On terraces and semis, rear extensions almost always trigger the Party Wall Act: new foundations excavated within 3m of a neighbour's structure (Section 6), or steelwork bearing into a shared wall (Section 2). Notices must be served one to two months before work starts depending on the work type. If neighbours dissent, a party wall award is agreed by surveyors. Routine, nothing to fear — but it must be programmed in from the start.

Extension design is heavily digital now, and the benefits show directly on site.

• 3D visuals that sell the scheme — to you and the planner. Seeing the kitchen-diner at real scale, with actual light levels, ends more arguments than any floor plan.
• Daylight modelling. We test how far natural light reaches into the plan before the design is fixed — the single most important check on any deep extension.
• Coordinated structural design. Steel sizes, pad foundations and drainage runs resolved with the structural engineer during design, not discovered on site.
• Measured surveys. Laser-measured existing drawings accurate to millimetres — because a 50mm error in the existing kitchen becomes a very expensive error in the new one.

The relationship with your rear extension architect will last the better part of a year. Choose carefully. These are the seven things Extendopedia tells clients to weigh up.

1. i.

   Relevant local experience

   A practice that knows Harrow's Victorian terraces, 1930s semis and conservation areas will spot the constraints — and the opportunities — that a generalist misses.

2. ii.

   Look at built extensions, not just renders

   Ask to visit a completed rear extension. Photos show composition; visits show light, craft, and how the junction between old and new has aged.

3. iii.

   Ask who will actually run your project

   The person at the pitch is rarely the person detailing the steel junction. Ask who will be on the tools week to week.

4. iv.

   Confirm ARB registration

   It protects you and signals genuine professional accountability. Verify it on the ARB register before you sign anything.

5. v.

   Clarify the scope in writing

   Concept only? Planning and building regulations? Tender and site support? Each shifts risk differently. Get it in writing.

6. vi.

   Understand the fee structure

   Ask whether the fee is fixed per stage or a percentage of construction cost — and what happens if planning needs amendments or the build overruns. A fixed fee agreed after a site visit means no surprises.

7. vii.

   Look for builder-facing experience

   The best extension architects think like builders: buildable details, realistic programmes, and drawings a contractor can price accurately.

Every project is different. The timelines below are typical for London and Harrow projects — we don't publish build costs because they depend on size, specification, ground conditions and access. After a site visit we confirm an exact, fixed figure before you commit to anything.

On top of build costs, allow for design fees, a contingency, and kitchen/bathroom fit-out which is often priced separately. What moves a project up or down the band: structural complexity (wider openings need bigger steel), glazing specification (slim-profile sliding doors cost serious money), ground conditions, and access constraints on tight London plots. For timeline, allow 8–14 weeks before construction for design and approvals. Every figure we quote is fixed after a site visit — never a guess from a table.

Real feedback from homeowners we've designed rear extensions for across Harrow and North West London. Rated 4.9 out of 5 from 87 verified reviews on Google and Houzz.