Side Return Extension London 2026: Costs, Planning & Design Guide
Turn your narrow side passage into a light-filled kitchen extension. 2026 costs, planning rules, RSJ beam requirements and everything else you need to know before starting.
Quick Answer
Side return extensions cost £48k–£110k+ in London (inc VAT) depending on width and depth. Almost all require planning permission. The RSJ structural beam is a critical cost most people miss.
£55k–£90k
Typical Cost
8–15m²
Space Gained
8–12 weeks
Check your specific property constraints
Free Property CheckWhat Is a Side Return Extension?
A side return extension fills the narrow passage that runs alongside terraced and semi-detached houses in London. This wasted strip of land, typically 1.0–1.5m wide and running the length of the ground floor, is built over to widen the kitchen or rear reception room.
Unlike a rear extension, a side return doesn't push into the garden. It uses land you already own but can't meaningfully use. The result is a wider, lighter ground floor that transforms the most common complaint about Victorian and Edwardian terraced houses: the narrow galley kitchen.
Most side returns are combined with the existing kitchen and rear living room to create a full-width open-plan space. Adding a rear extension at the same time creates what's called an L-shaped or wrap-around extension, maximising both width and depth. See our rear extension London 2026 guide for costs on the depth element.
At a glance
Side Return Extension Costs 2026
All costs below are inclusive of VAT at 20%. They cover structure, roof, glazing, basic internal finishes, and the RSJ structural beam. Kitchen and professional fees are listed separately.
| Width x Depth | Area | Budget (inc VAT) | Mid-Range (inc VAT) | High-Spec (inc VAT) |
|---|---|---|---|---|
| 1.0m × 5m | 5m² | £22k–£30k | £30k–£40k | £40k–£55k |
| 1.2m × 6m | 7.2m² | £30k–£42k | £42k–£56k | £56k–£75k |
| 1.2m × 8m (most common) | 9.6m² | £38k–£52k | £52k–£68k | £68k–£90k |
| 1.5m × 6m | 9m² | £36k–£50k | £50k–£66k | £66k–£88k |
| 1.5m × 8m | 12m² | £46k–£62k | £62k–£82k | £82k–£110k |
| Side return + rear (L-shaped) | 20m²+ | £80k–£105k | £105k–£145k | £145k–£200k+ |
Note: These costs cover structure and basic finishes. Add £8k–£30k+ for a new kitchen. Professional fees (architect and structural engineer) add a further £6k–£14k. See full breakdown below.
Cost per m² in 2026
| Spec Level | Cost per m² (inc VAT) | What it gets you |
|---|---|---|
| Budget | £3,800–£4,800 | Flat roof, polycarbonate rooflight, basic glazing, screed floor |
| Mid-range | £4,800–£6,200 | Roof lantern or structural skylight, bi-fold doors, tiled floor, underfloor heating |
| High-spec | £6,200–£8,500 | Full glass roof, aluminium sliding doors, heated polished concrete, premium finishes |
Side returns cost more per m² than a straightforward rear extension because the narrow geometry means more structural complexity per square metre. The RSJ beam, boundary wall construction and junction with the existing house all add cost relative to the floor area gained. For a broader view of London extension pricing, see our London extension costs guide for 2026.
Itemised Cost Breakdown
For a typical 1.2m × 8m side return (9.6m²) at mid-range spec:
Where the money goes
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Our AI calculates costs for your specific property, checks planning requirements, and tells you if permitted development applies.
Planning Permission: What You Actually Need
The Permitted Development Rule for Side Extensions
Under permitted development, a single-storey side extension can be built without planning permission provided it does not exceed half the width of the original house. In practice, this rule rules out most London terraces:
- A standard Victorian terrace is 4–5m wide. Half of that is 2.0–2.5m. The side passage is typically only 1.0–1.5m, so you might be within the width limit.
- However, PD for side extensions applies only if the extension does not front a highway. Most London terraced houses face a street, so the "side" elevation is visible from that street.
- Conservation areas remove PD rights for side extensions entirely. Around 30–40% of inner London homes are in a conservation area.
- Article 4 Directions in many inner London boroughs withdraw PD rights for side extensions even outside conservation areas.
The practical result: around 90% of London terraced house side return extensions need full planning permission. The good news is that approval rates are high when the scheme is well designed. For full rules, see our permitted development guide for 2026.
Planning Decision by Scenario
| Property type | Location | Route | Typical approval rate |
|---|---|---|---|
| Terraced house | Outside conservation area, no Article 4 | PD or full PP | 90%+ |
| Terraced house | Conservation area | Full PP required | 82–88% |
| Terraced house | Article 4 area | Full PP required | 85–90% |
| Semi-detached | Standard residential | PD possible if within half-width rule | 92%+ |
| Listed building | Any | Listed Building Consent + PP | Case by case |
What Planners Look For
Side returns are generally viewed favourably by London councils. The main planning considerations are:
- Subservience: The side return should appear subordinate to the main house, not the same width or height
- Materials: Match or complement the existing building, particularly in conservation areas
- Neighbour impact: Side glazing must not create overlooking or loss of light for the adjoining property
- Height: Must remain single-storey with eaves below the neighbour's first-floor window sill
Not sure if you need planning permission?
Check your property's planning requirements freeThe RSJ Beam: The Cost Most People Miss
A side return extension creates a new, wider room by removing the existing side wall between your house and the passage. This wall is load-bearing in most Victorian and Edwardian houses. Before it can come down, a structural steel beam, called an RSJ (Rolled Steel Joist), must be installed to carry the load above.
The RSJ is often excluded from early contractor quotes or buried in a "structural works" line item. Make sure you get a clear breakdown.
RSJ beam costs 2026 (inc VAT)
If the extension runs the full length of the ground floor and opens into the kitchen and reception room, you may need two beams, one at each end of the existing wall section. Budget accordingly.
Roof and Glazing Options
The roof and glazing choices have the biggest impact on how the finished space feels and how much natural light you get. They also account for 25–35% of total build cost.
Flat roof with skylights
£6k–£11kThe most common and cost-effective option. A warm flat roof (insulation above the deck) with one or two large structural rooflights delivers good daylight at lower cost. EPDM or GRP (fibreglass) waterproofing lasts 25+ years with minimal maintenance.
Glass / polycarbonate roof
£12k–£22kA full glass roof using aluminium glazing bars and double or triple glazed units floods the room with daylight. Requires solar-control glass to prevent overheating in summer. Building Regulations requires the glazed area to be less than 25% of the total floor area for thermal compliance, or a compensating SAP calculation.
Roof lantern
£3k–£8k (supply only)A roof lantern sits proud of a flat roof deck and draws light down into the centre of the room. Popular in high-spec side returns where planners or neighbours limit side-wall glazing. Often combined with a flat roof surround. See our roof lantern vs skylight comparison guide for more detail.
Side-Wall Glazing Options
The side wall of the extension faces the boundary with your neighbour. Glazing here brings light in from a second direction but can raise privacy concerns. Options:
- Clerestory windows: High-level fixed glazing above head height. Excellent light without overlooking. £2k–£5k.
- Full-height opaque glazing: Frosted or obscure glass gives light without a view. £3k–£7k.
- Bi-fold or sliding doors onto side: Only possible if the passage is wide enough to open doors (minimum 1.5m). Links to a side garden or utility area. £4k–£9k. See our bi-fold vs sliding door guide.
- Solid wall: If the neighbour is very close, a solid side wall with roof glazing only is often the cleanest solution.
Building Regulations
Building Regulations approval is required for all side return extensions regardless of whether planning permission is needed. Full guidance is in our building regulations for extensions guide.
Part A: Structure
Foundation design (strip, pad or raft depending on ground conditions), RSJ beam specification signed off by a structural engineer, roof structure, and connection to the existing building. London clay requires careful foundation depth, typically 1.0–1.5m.
Part L: Energy efficiency
The 2022 update (now in effect) tightened targets significantly. Wall U-value: 0.18 W/m²K, roof: 0.15 W/m²K, floor: 0.18 W/m²K. Glazing: 1.4 W/m²K. If glazing exceeds 25% of floor area, a SAP energy calculation is needed to demonstrate compliance.
Part F: Ventilation
Kitchen extraction is the main consideration. A cooker hood must discharge externally, not into a roof void. If the extension changes the kitchen layout, the extract duct route must be planned early, as it affects ceiling design and external appearance.
Part H: Drainage
Rainwater from the new roof must drain to a soakaway or the surface water system (not foul). If the kitchen sink is moving as part of the reconfiguration, the new drain run must be inspected by Building Control.
Part O: Overheating
The 2022 regulations added overheating requirements. If you specify a glass roof, the designer must demonstrate the extension won't overheat using the simplified method or dynamic thermal modelling. Solar-control glass (g-value below 0.35) typically satisfies this without further analysis.
Building Control fee: £900–£1,600 for a side return extension, depending on your borough. You can use your local authority Building Control or an approved private inspector (RICS-registered). Private inspectors are often faster but comparable in cost.
Party Wall Requirements
Side return extensions almost always trigger the Party Wall Act 1996 because they involve building on or close to the boundary with a neighbour. See our party wall agreement London 2026 guide for the full process.
Which party wall notices apply
Section 1 notice
1 month noticeRequired when building a new wall on the boundary line. The new side wall of your extension sits exactly on the boundary, so this notice is almost always needed. If the neighbour consents, work can start after one month.
Section 2 notice
2 months noticeRequired for works to an existing shared party wall or party fence wall. If you're cutting into or building off the existing wall between your house and next door, a Section 2 notice is needed two months before work starts.
Typical party wall costs
You pay both surveyors' fees. The process protects both you and your neighbour by documenting the condition of the neighbouring property before work starts and setting agreed working hours and method.
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Realistic Project Timeline
Months 1–2: Design and planning preparation
Appoint an architect, agree scope, survey the property, and prepare planning drawings. For conservation area schemes, allow an extra 2–4 weeks for pre-application discussions with the council.
Months 2–4: Planning application
Submit the planning application. Statutory determination period is 8 weeks but in practice inner London councils take 10–14 weeks. Serve party wall notices at the same time so the two-month notice period overlaps.
Month 4–5: Technical design and tendering
While planning is pending, the architect prepares technical drawings and the structural engineer designs foundations and the RSJ beam. Send these to three or four contractors for quotes. Allow four weeks for tenders to come back.
Month 5–6: Appoint contractor and pre-start
Sign contract, submit Building Regulations application, and order long-lead items such as bi-fold doors, roof lanterns, or structural steel. Good builders are often booked six to eight weeks ahead.
Months 7–9: Construction (8–12 weeks)
Foundations, blockwork, beam, roof, glazing, first fix (electrics, plumbing), insulation, plasterboard, second fix, floor, decoration. Building Control inspects at foundations, drainage, and completion stages. You can usually stay in the house throughout.
Total: 8–12 months from first architect meeting to completed extension. Allow 12–14 months if you're in a conservation area or if the party wall process is disputed.
5 Common Mistakes to Avoid
1. Not planning the kitchen layout before finalising the design
The structural beam position, soil pipe location, and where windows sit are all determined during the planning and design stage. If you leave the kitchen layout until after approval, you may find the beam is in the wrong place or there's no practical route for the cooker extraction. Plan the kitchen first, then design the structure around it.
2. Underestimating total costs by excluding kitchen, fees and contingency
Many quotes cover structure only. Add professional fees (10–15%), Building Regulations, party wall surveyors, a kitchen, and a 10–15% contingency. A £55k build quote can become £85k all-in. Know the true number before committing.
3. Serving party wall notices too late
Section 2 notices require two months before work starts. If your neighbour dissents, the party wall award process adds further time. Many homeowners serve notices only after planning is approved, adding two months they hadn't planned for. Serve notices as soon as plans are reasonably fixed.
4. Specifying a full glass roof without solving the overheating problem
A glass roof on a narrow south-facing extension can reach 40°C+ in summer. Part O (overheating) regulations now require evidence that the space won't overheat. Specify solar-control glass with a g-value below 0.35 and ensure adequate cross-ventilation before committing to a full glass roof.
5. Assuming permitted development applies when it doesn't
We see homeowners begin work on what they believe is a permitted development side return, only to receive an enforcement notice. Most London terraces need planning permission for side extensions. Check before starting, not after.
Frequently Asked Questions
How much does a side return extension cost in London in 2026?
A standard 1.2m × 8m side return costs £52k–£90k for structure and finishes (inc VAT). Add £8k–£30k for a kitchen and £6k–£14k for professional fees. Total all-in cost is typically £70k–£130k depending on spec. Wider or longer extensions and wrap-around L-shaped schemes cost more.
Do I need planning permission for a side return extension?
Around 90% of London terraced houses need planning permission for a side return. Permitted development rights exist in theory but are removed by conservation areas, Article 4 Directions, and the rule that the extension must not front a highway. Semi-detached properties have more flexibility. Always check with your local planning authority or an architect before starting.
How wide can a side return extension be?
The extension fills the existing side passage, which is typically 1.0–1.5m wide. Under permitted development, a side extension cannot exceed half the width of the original house. For planning permission applications there is no fixed width limit, but planners expect the extension to remain subordinate in scale to the main house.
What is an RSJ and why does it add to the cost?
An RSJ (Rolled Steel Joist) is the structural steel beam needed to carry the load where the existing side wall is removed. Without it, the upper floors would have nothing to sit on. The RSJ is specified by a structural engineer, supplied and installed by your builder. Cost including engineering, supply and installation: £3k–£7k.
Can I combine a side return with a rear extension?
Yes. Combining both creates an L-shaped or wrap-around extension that maximises ground floor space. This is one of the most effective ways to transform a terraced house. Expect combined costs of £80k–£200k+ depending on size and spec. You apply for planning permission for both at once, and only one set of professional fees applies.
How long does a side return extension take?
Construction takes 8–12 weeks. From first meeting with an architect to moving back into a finished kitchen-extension: 8–12 months. The main delays are planning (10–14 weeks for determination in London), party wall notices (2 months minimum), and waiting for a contractor start date. Serve party wall notices while planning is pending to overlap the processes.
Will my neighbour be affected by the build?
Yes, and the Party Wall Act protects both of you. You must serve notice at least one to two months before work starts, depending on the notice type. Your neighbour can appoint their own surveyor (at your expense) to review the works. A schedule of condition documents the state of their property before work begins, so any damage can be assessed afterwards.
How much value does a side return extension add?
In most London locations, a side return extension adds £60k–£100k to property value, which typically exceeds the all-in build cost. The open-plan kitchen-diner layout is one of the most desirable features for buyers. Return on investment is generally 110–140% in inner London. Value uplift is lower in outer London where house prices are lower relative to build costs.
Summary
A side return extension is one of the most transformative projects you can do to a Victorian or Edwardian terraced house in London. For £65k–£110k all-in (structure, kitchen, and fees), you convert a dark galley kitchen into a full-width, light-filled room that adds more value than it costs.
Planning permission is required for most London terraces, but approval rates are high when the design is well-considered. Allow 8–12 months from first meeting to finished result. Serve party wall notices early, plan the kitchen layout before design is fixed, and budget for the RSJ beam as a separate cost item.
For a broader comparison of all extension types and their costs, see our London extension costs 2026 guide. For ROI across different extension types, see our extension ROI London 2026 guide.
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