1930s Semi Extension London 2026: Costs, Planning & Types
The 1930s semi-detached is the most extended house type in outer London. Wide plots, hipped roofs, and generous gardens make them ideal candidates — but each extension type has its own planning route, costs, and structural considerations.
Quick Answer
1930s semi extensions cost £45k–£150k+ inc VAT depending on type. A rear extension costs £55k–£120k, a side extension £45k–£85k, a hip-to-gable loft £55k–£90k, and a wrap-around £80k–£150k. Most rear extensions qualify for permitted development (3m) or prior approval (up to 6m). Side extensions and hip-to-gable lofts have their own rules.
£55k–£120k
Rear extension
£55k–£90k
Hip-to-gable loft
£80k–£150k
Wrap-around
Check your specific property constraints
Free Property CheckWhy 1930s Semis Dominate Outer London Extension Work
The 1930s semi-detached is the defining house type of outer London. Zones 3 to 5 — Finchley, Ealing, Bromley, Sutton, Croydon, Walthamstow, Ilford — are lined with them. They were built in enormous numbers during the interwar suburban expansion, when London's Underground and surface rail network pushed commuter routes far into the surrounding countryside.
These houses are popular extension candidates for three structural reasons. First, the plots are wide — typically 6–8m at the building line, with a usable side passage between the two semis. Second, rear gardens are generous, often 15–20m deep, which gives ample room for a rear extension without losing the garden. Third, the hipped roof — where all four sides slope to the walls — creates an underused loft void that is well suited to conversion.
Unlike a Victorian terrace, which typically sits on a narrow plot with a small side return and a solid brick party wall, a 1930s semi offers four distinct extension options: rear, side, hip-to-gable loft, and wrap-around. Most households end up doing two or three of these over the life of their ownership.
The Typical 1930s Semi Layout — And What Is Wrong With It
The standard three-bedroom 1930s semi has a recognisable plan. Ground floor: front reception room (the "front room"), second reception room behind it, and a small galley kitchen at the rear. A side porch or lean-to utility often exists. Upstairs: three bedrooms and a bathroom, with a small box room at the front. Total footprint roughly 55–65m².
What the original layout gets wrong for modern life
A well-executed extension programme on a 1930s semi can increase the total floor area from around 65m² to 110–130m², transform the ground-floor layout into an open-plan kitchen-diner with garden connection, add a fourth bedroom or home office, and — with a hip-to-gable loft — create a fifth room with an en-suite. The transformation is significant, and the costs are generally recovered in increased property value.
The Four Classic Extensions for a 1930s Semi
1. Rear Extension: The Kitchen-Diner
The rear extension is the first project most 1930s semi owners undertake. The goal is almost always the same: knock through the rear reception room and kitchen, extend into the garden, and create an open-plan kitchen-diner with bi-fold or sliding doors onto the garden.
Because 1930s semis are semi-detached, permitted development allows a single-storey rear extension up to 3m from the original rear wall without any application. The Neighbour Consultation Scheme (prior approval) extends this to 6m from the original rear wall — enough for a very generous kitchen-diner. Most 1930s semi rear extensions fall into the 3–5m depth range, which means prior approval is common.
Rear extension: planning routes
Rear extension costs (inc VAT) — 2026
All-in includes construction, architect, structural engineer, building regulations, and 10% contingency. Does not include new kitchen (add £12k–£40k+).
2. Side Extension: The Fourth Bedroom or Enlarged Kitchen
The side passage on a 1930s semi — typically 900mm to 1.5m wide — is one of the most underused spaces in the building. A side extension builds over this gap to create additional ground-floor space: an enlarged kitchen, a utility room, a downstairs WC and boot room, or (if two storeys) a fourth bedroom above with a ground-floor addition below.
Under permitted development, a side extension is allowed as long as it is single storey and no wider than half the width of the original house. For a typical 1930s semi with a house width of 6–8m, half the original width means the extension can be up to 3–4m wide — far wider than the original side passage. Most side extensions on 1930s semis are 2.5–3.5m wide, well within PD limits.
A two-storey side extension always requires full planning permission. Most London boroughs will approve a well-designed two-storey side extension on a 1930s semi, but they will look carefully at the setback from the boundary and the design relationship to the shared roofline.
Side extension costs (inc VAT) — 2026
3. Hip-to-Gable Loft Conversion: Full-Width Loft Room
The hipped roof is the defining architectural feature of the 1930s semi. All four sides of the roof slope downward to the eaves — including the two end slopes (the "hips"). On a semi-detached house, only one end is exposed (the non-shared gable end). This hip slope eats into the available loft floor area significantly.
A hip-to-gable conversion removes that sloping hip and replaces it with a vertical gable wall, creating a full-width rectangular loft floor. The result is typically 30–40% more usable floor area than a standard loft conversion on the same house. The new gable wall is usually built in matching brick or render to complement the existing house.
Hip-to-gable costs (inc VAT) — 2026
Cost range depends on size of roof, number of rooms, spec of finishes, and whether a full staircase reconfiguration is required.
4. Wrap-Around Extension: Rear and Side Combined
A wrap-around combines a rear extension and a side extension into a single L-shaped project. On a 1930s semi, this typically means extending 4–5m to the rear and 2.5–3.5m to the side simultaneously. The result is a large open-plan ground-floor living space — typically 35–50m² of new floor area — that transforms the entire rear half of the ground floor.
Because the rear and side elements are separate structures, the planning position for each applies independently: the rear element may qualify for prior approval up to 6m, and the single-storey side element may qualify for permitted development up to half the house width. However, the interaction between them — and the overall impact on the building — often means a single planning application is cleaner and more reliable.
Wrap-around costs (inc VAT) — 2026
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Hip-to-Gable: What Councils Look For
Hip-to-gable loft conversions on semi-detached houses can technically fall within permitted development — specifically, loft additions up to 40m³ on a semi-detached house are permitted development provided they do not exceed the existing ridge height, are set back from the eaves, and use materials that match the existing roof.
However, in practice, most London boroughs interpret the hip-to-gable as changing the shape of the original roof in a way that falls outside the standard loft PD criteria, or the works exceed 40m³ once a rear dormer is added. Full planning permission is frequently required, and most architects will recommend applying for it rather than relying on a marginal PD claim.
What councils assess for hip-to-gable applications
The new gable must not exceed the existing ridge height. On a 1930s semi with a standard pitched roof, this is not usually an issue — the gable wall terminates at or below the ridge.
The new gable wall and any dormer should use materials that match the existing house. 1930s semis are predominantly London stock brick or red brick — your architect should specify a matching brick, and the council may require a sample panel if they are not confident of the match.
The hip slopes on a pair of 1930s semis are often symmetrical — they mirror each other. Converting one half to a gable creates an asymmetry. Most councils accept this as a normal feature of London suburbia, particularly where hip-to-gable conversions are common in the area. If the neighbours have already done one, approval is generally straightforward.
If the property is in a conservation area, hip-to-gable conversions are much harder to achieve. The roofline of 1930s semis is often considered a positive contribution to the conservation area character. Expect a refusal or conditions requiring a very discreet design.
Most hip-to-gable applications include a rear dormer to maximise the usable floor area. The dormer should be set back from the eaves (typically at least 200mm), be smaller than the main roof slope it sits within, and use a flat or shallow-pitched roof. Councils look at the dormer size relative to the roof plane — large dormers on small roof slopes are routinely refused.
Permitted Development Rights for 1930s Semis
1930s semi-detached houses fall into the same PD category as any other semi-detached residential dwelling. The key limits to know:
| Extension type | PD limit | Prior approval limit |
|---|---|---|
| Rear extension (single storey) | 3m from original rear wall | 6m from original rear wall |
| Side extension (single storey) | No wider than half the original house width | No prior approval route — needs full planning if two storey |
| Loft addition (semi) | Up to 40m³ additional volume | No specific prior approval route — planning required if outside PD |
| Overall height | Must not exceed original ridge height | — |
Article 4 directions in outer London
Several outer London boroughs have introduced Article 4 directions that remove some or all permitted development rights in specific areas. Boroughs where Article 4 directions commonly affect 1930s semis include Bromley (various conservation areas), Sutton, Croydon (parts of South Croydon and Purley), and Ealing. If your property is subject to an Article 4, you will need full planning permission even for a 3m rear extension.
Check the planning portal for your borough, or use our AI to look up your address — it identifies Article 4 areas and conservation area status automatically.
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Check now — it's free →Planning Permission: Design Guidance for 1930s Character
Where a planning application is needed — for a two-storey side extension, a hip-to-gable loft, or a rear extension beyond 6m — London councils assess the design against the existing character of the house and street. 1930s semis have a distinct set of character features that councils expect new extensions to respect.
Brick matching
1930s semis in London are predominantly built in London stock brick (yellow-grey) or Fletton brick (orange-red). Architects should source a matching second-hand or purpose-made brick. Councils increasingly require a physical sample panel on site before proceeding if the brick match is not clear from drawings.
Window proportions
1930s semis typically have vertically proportioned windows with shallow reveals and decorative brick detailing above. Extensions — particularly two-storey side additions — should continue the vertical rhythm. Horizontal bands of glazing can look incongruous and attract objections from neighbours and design officers alike.
Flat roofs on rear extensions
Single-storey rear extensions with flat roofs are well-established at the rear of 1930s semis and are rarely refused solely on roof form. However, in conservation areas, a lean-to or low-pitched roof may be required. At the side, visible flat-roofed additions can attract objection — a hipped or pitched roof that reflects the original roofline is often preferred.
Subservience
A key design test councils apply is whether the extension appears subservient to the original house. For a two-storey side extension this means: it should be set back from the front elevation, its ridge height should be lower than the original ridge, and the materials should be complementary without being identical. Matching exactly can look odd; contrasting too dramatically can be refused.
2026 Cost Summary: All Extension Types
All figures include VAT at 20%. Construction cost is the build contract only. All-in adds architect, structural engineer, building regulations, and a 10% contingency. Professional fees for a typical project run to £5,000–£12,000 depending on complexity.
| Extension type | Typical size | Construction | All-in |
|---|---|---|---|
| Rear (3m, single storey) | 15–18m² | £45k–£65k | £55k–£78k |
| Rear (4–5m, single storey) | 20–30m² | £60k–£90k | £72k–£108k |
| Rear (5–6m, prior approval) | 30–40m² | £80k–£115k | £96k–£138k |
| Side (single storey) | 10–20m² | £36k–£65k | £45k–£78k |
| Side (two storey) | 20–35m² | £72k–£115k | £86k–£138k |
| Hip-to-gable loft | 20–35m² | £45k–£72k | £55k–£86k |
| Hip-to-gable + dormer + en-suite | 30–45m² | £58k–£88k | £70k–£105k |
| Wrap-around (rear + side) | 35–55m² | £68k–£118k | £82k–£142k |
Construction cost per m² in London 2026
All figures inc VAT at 20%. Outer London (zones 3–5) typically runs at the lower end of the London range. Inner London (zones 1–2) runs 10–15% higher. Loft conversions and two-storey extensions are priced differently per m² — the table above uses appropriate ranges for each type.
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Structural Specifics: What Makes 1930s Semis Different
Understanding the structure of a 1930s semi avoids surprises during construction. These houses were built to a generally consistent standard, but they differ from both Victorian terraces and post-war homes in a few important ways.
Cavity walls — early and inconsistent
The 1930s was a transitional period for wall construction. Some 1930s semis were built with cavity walls — two leaves of brick with a 50mm air gap — which was an innovation of the period. Others, particularly early 1930s builds and those in certain regions, were built with solid 9-inch brick walls. The wall type matters for extending: cavity walls require careful tying of the new extension to the existing structure. Your structural engineer will confirm the wall type during design. Neither type presents a major problem for extension work, but the connection detail differs.
Shallow strip foundations
1930s strip foundations are typically shallower than modern standards — often 600–900mm below ground level. New extensions require foundations at an appropriate depth for current building regulations (typically 1m+ in London clay). Your engineer will specify the new foundation depth and the junction detail where new meets old. In some cases, if the existing foundations are very shallow, underpinning of the existing rear wall may be required before the structural opening is formed — this adds cost (£5,000–£15,000) and should be identified at design stage through a structural survey.
London clay and trees
Outer London is dominated by London clay — a shrinkable clay that expands when wet and contracts in dry conditions. Suburban 1930s gardens frequently contain mature trees (oak, ash, poplar, silver birch) planted during the same period as the houses. Tree roots can extend far beyond the canopy and cause clay shrinkage at depth. If any trees are within 10–15m of your proposed extension, your structural engineer will assess the tree-root influence zone and may specify deeper foundations (sometimes 1.5–2.5m) or pile foundations if a conventional strip is not viable. The council may also require a tree survey and protection plan as part of the planning application.
The party wall between the two semis
The shared wall between a 1930s pair of semis runs the full height of the building — ground floor to ridge. It is a single wall shared by both owners. Any work that cuts into, builds off, or excavates close to this wall requires a Party Wall Award under the Party Wall Act 1996. For a rear extension, this is triggered by excavating foundations within 3m of the neighbour's structure — which is almost always the case. For a hip-to-gable loft, the work is directly to the party wall at the gable end. Budget £800–£1,400 per neighbour for party wall surveyor fees.
Party Wall: Side Extensions Are the Critical Case
For a rear extension, the party wall issue is usually straightforward: serve notice on the attached neighbour for foundation excavations, allow the 2-month notice period, and proceed with an Award if they dissent. Most neighbours consent without dispute for a standard rear extension.
Side extensions along the shared boundary wall are more complex. The new wall of the side extension often runs along or very close to the shared boundary. If the new wall is built right on the boundary line, it becomes a party fence wall under the Act. If the foundations for the side extension are within 3m of the neighbour's structure, notice is required under Section 6. If any part of the work is directly to the party wall — such as where the hip-to-gable conversion meets the shared gable wall — Section 2 notice is required.
Serve party wall notice early. The 2-month statutory notice period starts from the date the notice is served, not from when your neighbour responds. If they dissent or ignore it, the Award process takes a further 4–8 weeks. For a hip-to-gable, where the neighbour's attached loft and party wall are directly involved, expect the party wall process to take 3–4 months. Start as soon as you have outline drawings.
Typical cost: £800–£1,400 per adjoining owner. A 1930s semi has one party wall neighbour, so budget £1,000–£1,500 for this. If there are trees or an existing loft on the neighbour's side, appoint an experienced party wall surveyor early.
Common Mistakes on 1930s Semi Extensions
Underestimating hip-to-gable planning complexity
Hip-to-gable conversions are often marketed as "usually permitted development." In practice, London boroughs are inconsistent, and many require a full planning application. Submitting a lawful development certificate application for a marginal case can delay a project by 8 weeks, only to be refused — at which point a full planning application is needed anyway. Discuss the PD route with your architect and check local precedents in your borough before deciding. If in doubt, a planning application is more reliable.
Over-extending the side and losing neighbour's light
A two-storey side extension on a 1930s semi can significantly reduce natural light to the neighbour's ground floor windows. The non-attached neighbour on the other side of the new extension — who may have a kitchen or living room facing the boundary — may object on the grounds of loss of daylight. The BRE daylight and sunlight guidelines are a material consideration. Your architect should run a daylight assessment for a two-storey side extension. Refusals for over-dominant two-storey side extensions on 1930s semis are not uncommon in boroughs including Barnet, Ealing, and Bromley.
Not identifying the original rear wall correctly
Permitted development depth is measured from the original rear wall of the house, not from any existing lean-to, garage, or previous extension. Many 1930s semis have had a small lean-to, toilet outbuilding, or single-storey addition built at the rear over the decades. If a previous extension has used some of the permitted development allowance, your available depth is reduced accordingly. Check the original building footprint on your council's planning portal before finalising your scheme.
Ignoring trees in the rear garden
Mature trees in the rear garden of a 1930s semi are a very common complication. If the tree is protected by a Tree Preservation Order (TPO) or in a conservation area, any works affecting the roots require council consent. Even without a TPO, a tree root survey may be required by building control or your structural engineer. The cost of deeper foundations to avoid root zones can add £8,000–£20,000 to a rear extension budget. Identify trees and their protection status at the start of the project.
Doing the loft before the rear extension
If you plan both a rear extension and a hip-to-gable loft conversion, sequencing matters. A rear extension creates significant disruption to the rear ground floor. A loft conversion creates disruption to the roof and first floor. Doing them together (or immediately sequentially) reduces the total disruption period and allows the builder to price the combined scaffolding, roof access, and structural opening as a single contract. Doing them years apart means two separate mobilisations, two separate scaffolding erections, and usually higher combined costs.
Frequently Asked Questions
How much does it cost to extend a 1930s semi-detached house in London in 2026?
Costs inc VAT: rear extension £55k–£120k all-in depending on depth and size; side extension £45k–£85k; hip-to-gable loft conversion £55k–£90k; wrap-around (rear and side combined) £80k–£150k. Construction cost per m² runs £3,000–£5,500 in London depending on spec. Add professional fees (architect, structural engineer, building regulations) of £5,000–£12,000 on top of construction.
Do I need planning permission to extend a 1930s semi-detached house?
For a single-storey rear extension: no application needed up to 3m depth (permitted development), and prior approval covers up to 6m. For a side extension: single storey up to half the house width is permitted development; two storey needs planning. For a hip-to-gable loft: technically PD under 40m³ but many London boroughs require full planning. Conservation areas and Article 4 areas remove PD rights and require full planning for all extensions.
What is a hip-to-gable loft conversion and why is it used on 1930s semis?
A hip-to-gable conversion removes the sloping hip roof at the exposed end of a semi-detached house and replaces it with a vertical gable wall, creating a full-width rectangular loft floor. It is ideal for 1930s semis because the hipped roof is the defining feature of the era, and the conversion creates 30–40% more usable loft floor area than a standard Velux conversion on the same house. Most also include a rear dormer to maximise headroom.
What is the permitted development width limit for a side extension?
Under permitted development, a single-storey side extension must be no wider than half the width of the original house. For a typical 1930s semi with a house width of 6–8m, this allows an extension of up to 3–4m wide — well beyond the original side passage width of 900mm–1.5m. Two-storey side extensions are not permitted development and require full planning permission.
Do 1930s semis have cavity walls?
Some do, some do not. The 1930s was a transitional period for wall construction. Early 1930s builds and many regional variations were still built with solid 9-inch brick walls. Later 1930s and more progressive builders used cavity walls — two leaves of brick with a 50mm air gap. Your structural engineer or a wall type inspection will confirm which you have. Both wall types are workable for extensions, but the connection detail and insulation approach differ.
Do I need a party wall agreement for a side extension on a 1930s semi?
Yes, almost certainly. A side extension along the shared boundary wall triggers the Party Wall Act 1996 in several ways: if the new wall is built on or right to the boundary, if the foundations are within 3m of the neighbour's structure, or if any work is directly to the party wall itself. For a hip-to-gable conversion, the work is directly to the shared gable wall. Budget £800–£1,400 for party wall surveyor fees. Serve notice at least 2 months before work starts.
What areas of outer London have lots of 1930s semis?
1930s semi-detached houses dominate zones 3–5 across all directions out of central London. Well-known concentrations include Finchley, Barnet, Edgware, Harrow (north), Ealing, Hanwell, Greenford, Northolt (west), Bromley, Beckenham, Croydon, Sutton, Cheam (south), Ilford, Romford, Walthamstow, Chingford (east). The interwar suburban expansion was driven by Underground and surface rail electrification that made these areas commutable for the first time.
Is it worth extending a 1930s semi-detached house?
In most outer London locations, yes. A well-executed rear extension and loft conversion can increase the floor area by 50–70% and typically adds more value than the cost of construction. The 1930s semi's wide plot, generous garden, and hipped roof make it one of the most extensible house types in London. The key is sequencing: if you plan multiple extensions, combining or closely sequencing them reduces total mobilisation costs.
Summary
The 1930s semi-detached is the most extensible common house type in outer London. The combination of a wide plot, a usable side passage, a generous rear garden, and a hipped roof gives you four distinct extension options that no Victorian terrace or post-war house can match in the same combination.
The rear extension and loft conversion are the most transformative individually. Together, they can add 50–70% to the floor area of the original house. The side extension adds ground-floor utility space or a bedroom above. The wrap-around combines rear and side in one project, which is usually the most cost-effective way to do both.
Start with the rear extension if you need to prioritise: it has the clearest planning route, the highest day-to-day impact, and the best cost-to-value ratio. Address the hip-to-gable loft once the ground floor is done, and plan both together if your budget allows.
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