Area guide · EN4
House Extensions in Cockfosters (EN4)
Planning a house extension in Cockfosters? You've found the local firm — The Extension Company is based right here in Cockfosters, and EN4 is where we live, design and build. Most single-storey rear extensions here cost £50,000–£100,000 to build, your application is decided by Barnet or Enfield depending on which side of Cockfosters Road you live, and the most-approved project type in EN4 right now is the hip-to-gable loft conversion. Book a free site visit — for us it's a walk down the road.
Live planning data
What Barnet and Enfield are actually approving right now
We track every residential planning decision across North London — and because Cockfosters is home, we watch EN4 more closely than anywhere else. In the most recent decision period, these are real schemes approved on Cockfosters' streets, all decided by Barnet:
- Heddon Court Avenue, EN4 — rear dormer with Juliette balcony, front rooflight and a new side gable window (approved subject to conditions)
- 46 Haslemere Avenue, EN4 — hip-to-gable roof extension with replacement rear dormer (lawful development)
- 9A Rosslyn Avenue, EN4 — double hip-to-gable extension with rear dormer (lawful development)
Six EN4 schemes went through Barnet in that recent window, and the pattern is hard to miss: hip-to-gable loft conversions dominate Cockfosters approvals. That's no accident — the avenues here are lined with hipped-roof 1930s houses, and converting the hip to a gable with a rear dormer is the single highest-value move this stock allows. Two of the three schemes above went through as lawful development, the quietly confident route that doesn't even need a full application. And on the other side of Cockfosters Road? Enfield approved 113 residential schemes borough-wide in the same recent two-week window — both of Cockfosters' councils are saying yes to well-designed extensions.
Source: Barnet's and Enfield's published planning registers. These schemes illustrate the local pattern of approvals — none of them are ours, and the outcome of any application always rests on its own particulars.
Local knowledge
Cockfosters' housing stock — and what works on each
Cockfosters is a tube suburb in the purest sense. The Piccadilly line terminus opened in 1933, and the wide avenues of hipped-roof semis and detached homes followed it — generous plots, side passages, deep gardens, with Trent Park on the doorstep. Here's how we'd extend each part of it.
The 1930s avenues (Heddon Court Avenue, Haslemere Avenue, Rosslyn Avenue and their neighbours). Classic tube-boom stock: bay-fronted, hipped-roof, and built for two moves. The hip-to-gable loft with rear dormer (£60,000–£100,000 here) adds a double bedroom and en-suite without touching the garden — exactly what the approvals list shows going through, street after street. The single-storey rear extension (£50,000–£100,000) opens the kitchen across the full width of the plot; under the larger home rules a 6m depth is achievable with prior approval. Plenty of owners phase both a year or two apart.
Detached Cockfosters — towards Trent Park and the Hadley Wood border. The bigger detached homes carry the area's premium. Permitted development stretches further on detached houses — 4m rear as standard, up to 8m with prior approval — and the plots take double-storey rear extensions (£85,000–£180,000) and wrap-arounds (£80,000–£160,000) comfortably. With Trent Park so close, some addresses sit near protected land, so we check constraints before anyone falls in love with a scheme.
The two-borough streets. Cockfosters is one of the few North London neighbourhoods split down the middle: west of Cockfosters Road your application goes to Barnet; on the Trent Park side it goes to Enfield. The national permitted development rules are identical, but validation checklists, local policies and case officers are not. Because we're based here, we submit to both councils routinely — whichever side of the road you're on, we already know the drill.
Every street is different. That's the point.
We design to Cockfosters's stock and to what the council demonstrably approves — not to a template. A free site visit tells you what your house can take.
Get my free estimateHonest numbers
What a house extension costs in Cockfosters in 2026
Cockfosters is a premium pocket — build costs typically run 10–15% above the North London baseline, reflecting larger houses and higher finish expectations. These are realistic all-in build ranges, excluding VAT, professional fees and kitchen fit-out:
| Extension type | Typical Cockfosters cost | On-site time | Usual planning route |
|---|---|---|---|
| Side-return extension | £40,000 – £75,000 | 2–3 months | Often permitted development |
| Single-storey rear | £50,000 – £100,000 | 3–4 months | Often permitted development |
| Kitchen extension (incl. mid-range kitchen) | £60,000 – £125,000 | 3–4 months | Often permitted development |
| Hip-to-gable loft + rear dormer | £60,000 – £100,000 | 6–10 weeks | Usually permitted development |
| Wrap-around extension | £80,000 – £160,000 | 4–6 months | Full application |
| Double-storey rear | £85,000 – £180,000 | 5–7 months | Full application |
| Garage conversion | £20,000 – £40,000 | 4–6 weeks | Usually permitted development |
The North London baseline runs £2,800–£4,200 per square metre; in Cockfosters budget £3,100–£4,700/m². Work that through for a 20m² rear addition and you arrive at £65,000–£90,000 of build cost. Once we've walked your property on the free site visit, your quote comes back fixed and broken down line by line — and the figure on the contract is the figure on the final invoice.
Permissions, handled
Planning permission in Cockfosters — what you need to know
Start with the council question. West of Cockfosters Road is Barnet; the Trent Park side is Enfield. It changes where your forms go and whose local policies apply — we confirm it from your address on day one.
Most Cockfosters extensions don't need a full application. Under permitted development a semi can push a single-storey rear extension 3m out — detached homes get 4m — and the larger home prior-approval process stretches those depths to 6m and 8m respectively. Loft conversions enjoy a generous volume allowance — the reason EN4's approvals are stacked with hip-to-gable schemes done as lawful development.
Where you do need permission: double-storey rears, wrap-arounds, flats and maisonettes, and homes where previous owners have already used up permitted development rights. Whichever side of the road you're on, plan for roughly eight weeks from validation to decision — both councils work to the same householder target.
Party walls: most of the avenue stock is semi-detached, so the Party Wall Act applies to the majority of projects here — budget £1,000–£2,500 per affected neighbour if surveyors are appointed, and start the conversation early. We serve the notices and manage the process.
Paperwork that pays: wherever a project proceeds under permitted development, we recommend a Lawful Development Certificate — it makes your extension bulletproof when you come to sell, and at Cockfosters prices that matters.
How it works
How it works
- Free site visit — we're based in Cockfosters, so we can usually walk your property within days, talk through what you want, and give straight answers on feasibility, cost and which council you're dealing with.
- Design & drawings — we draw the scheme up properly: scaled plans and 3D visuals shaped around your EN4 house and the way your family actually uses it.
- Planning, handled — we prepare and submit to Barnet or Enfield, whichever side of the road you're on, and manage it to decision.
- Fixed-price build — the build then passes to our vetted team, who carry it through building control sign-off and leave the place tidy on handover day.
Our work
Recent work near Cockfosters
Our build team's recent North London projects include a full house transformation taking a family home from 120m² to 360m², a 55m² garden room with kitchen and bathroom, and an Edwardian conversion in Finchley. When we come round, just ask — we'll bring along the projects that sit closest to your own brief.
Questions, answered
Cockfosters extension FAQs
Which council decides my extension in Cockfosters?
It depends on your side of Cockfosters Road — west of it you're in Barnet; the Trent Park side is Enfield. The split changes the forms, the local policies and the case officers, not your underlying rights as a homeowner. We confirm your council from the address before any design work starts.
How much should I budget for an extension in EN4?
Around £50,000–£100,000 for a single-storey rear build, £60,000–£100,000 for a hip-to-gable loft, and £85,000 upwards for double-storey schemes — excluding VAT and fees. Cockfosters builds at roughly £3,100–£4,700/m², about 10–15% above the North London norm.
What's the most popular extension on Cockfosters' 1930s houses?
The recent EN4 approvals answer that: hip-to-gable lofts with rear dormers, on Haslemere Avenue, Rosslyn Avenue and Heddon Court Avenue among others. The hipped roofs here convert beautifully into a full extra bedroom and en-suite.
Can I build a 6m rear extension in Cockfosters?
On most semis, yes — via the larger home prior-approval route (up to 8m on detached homes), which involves notifying the neighbours rather than making a full application. We'll confirm your eligibility at the site visit.
Does living near Trent Park restrict what I can build?
The park itself doesn't remove your permitted development rights, but some addresses near protected land carry extra constraints. We check your address against both councils' conservation and constraint maps before designing — it takes minutes and avoids expensive surprises.
Are you really based in Cockfosters?
Yes — this is our home town and our base. It's why we track EN4 decisions street by street, know both councils' quirks first-hand, and can usually get to your property for a free site visit within days.
Nearby areas we cover
What clients say
Rated 5.0 on Google
“Used The Extension Company for a loft conversion on our semi detached in Barnet and couldn't be happier. They explained what was possible at the first visit, sorted the planning side, and gave a fixed price with no surprises at the end.…” read the full review
“Really impressed with the entire project. The side extension and conservatory have been finished to a very high standard and the team made the process straight forward and stress-free. It’s transformed the space and added so much value, would definitely recommend.”
“Brilliant experience with the extension company, transparent from the word go. Completed a double story extension for me and I couldn’t be more pleased. Thank you”
“I got the extension company to do a bathroom refurb for us a January they done a great job they managed to fit in a large shower as well as a bath”
Reviews shown word for word from our Google profile. Read all 8 reviews on Google
Free site visit
Ready to see what's possible on your street?
Free site visit anywhere in Cockfosters, honest feasibility advice and a fixed, itemised price — design, planning and build handled end to end by the Pine Grove team.
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