Whole-house refurbishment with rear extension and loft conversion
Hill Crescent
Totteridge, North London N20
Double storey extensions in North London
Double Storey Extensions
Living space below and a bedroom or bathroom above — the most floor area per pound of any extension.
A double storey extension adds rooms on two floors — typically a larger kitchen or living space at ground level with a bedroom, bathroom or study above. Because the foundations, roof and scaffold are shared between two floors, it is the most cost-efficient way to add floor area, and it is often the answer for a growing family that needs both a bigger kitchen and another bedroom.
Double storey rear extensions are possible under permitted development within tight limits; side and most rear schemes in North London need a planning application, which we prepare and manage.
Who this suits
- Semi-detached and detached houses with room beside or behind the house
- Families who need a fourth or fifth bedroom as well as a bigger kitchen
- Homes where the roof does not suit a loft conversion
- Projects where the value case depends on adding a bedroom, not just living space
Planning rules for two-storey extensions
Under permitted development a two-storey rear extension may project no more than 3 m, must be at least 7 m from the rear boundary, must not exceed the height of the existing house, must match the roof pitch, and must have obscure-glazed, non-opening upper-floor side windows. Two-storey side extensions are never permitted development, and on designated land no two-storey extension is. In practice most double storey extensions in Barnet, Harrow and Hertsmere go through a householder application, where the tests are the impact on neighbours' light and outlook (the 45-degree rule), a subordinate appearance (set back from the front, roof lower than the main ridge) and matching materials. A side extension that leaves at least 1 m to the boundary at first floor is a common council expectation to avoid a "terracing effect".
Source: Planning Portal — extensions, planning permission; GPDO 2015 Schedule 2 Part 1 Class A.
Structure and roof
Two storeys mean deeper foundations designed for the load, a first-floor structure tying into the existing house, and a roof that either continues the main roof (hipped or gabled to match) or sits lower as a subordinate wing. Where the extension is over an existing single-storey part of the house, the old foundations are checked and usually underpinned or replaced. Upstairs, the new bedroom is planned around the stairs and landing that already exist; sometimes a short new corridor is the difference between a good plan and a compromised one.
Cost and value
A double storey extension typically costs 50–60% more than the same footprint as a single storey, for twice the floor area. Adding a bedroom with an en suite is one of the more reliable ways to add value in North London's family-house streets, provided the garden and parking are not compromised. We give a view on the likely value effect at feasibility.
Cost guide · figures current as of September 2026
What double storey extensions cost in North London
| Item | Typical range | Notes |
|---|---|---|
| Double storey rear or side extension, 40–60 m² total | £120,000 – £220,000 | |
| Bathroom or en suite, supply and fit | £8,000 – £20,000 | |
| Kitchen, supply and fit | £15,000 – £45,000 | |
| Design, engineering, planning and Building Regulations | 8 – 12% of build cost |
North London market ranges excluding VAT, fees, kitchen and bathrooms. Roof form, foundation depth and glazing are the main variables.
What could your double storey extensions project cost?
Use our project calculatorSources: Architecture for London — extension costs 2026; Checkatrade — double storey extension costs. Market guides only — not a Milkov & Son quotation.
How long it takes
- 01Design and planning4–6 months
- 02On site14–24 weeks
Stages overlap where they can. We agree a programme in writing before work starts — see how a project runs.
Frequently asked questions
Can I build a double storey extension without planning permission?
Only a two-storey rear extension within strict limits: 3 m deep, 7 m from the rear boundary, roof pitch matching, no higher than the house, and not on designated land. Two-storey side extensions always need planning permission.
How much does a double storey extension cost in North London?
Typically £120,000 to £220,000 for 40–60 m² of total floor area in 2026, excluding VAT, fees, kitchen and bathrooms — roughly 50–60% more than a single storey of the same footprint.
How long does a double storey extension take?
14 to 24 weeks on site after four to six months of design and planning. Living in the house is possible but harder than with a single storey; many clients move out for the structural phase.
Read before you build
Extensions
Single-Storey vs. Double-Storey Extensions: Which Is Right for You?
A double storey extension costs 50–60% more than a single storey of the same footprint for twice the floor area, but almost always needs planning permission. Cost, planning, structure and which suits your house.
1 May 2025 · 6 min read
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Should You Extend Your Home Upwards, Outwards, or Downwards?
When families outgrow their homes or property owners want to add value, extending a house becomes an attractive option. For many homeowners in the UK, the…
19 August 2025 · 6 min read
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What’s the Best Home Extension for a Growing Family?
For many families, the home that once felt perfectly spacious can quickly start to feel tight as children grow, needs change, and daily routines demand more…
26 August 2025 · 6 min read
Double Storey Extensions across North London
Based in Hendon, we work across North London and the Hertfordshire border. Each area page covers the local housing stock, planning context and the kind of work we do there.
All areasRelated services
House Extensions
Rear, side return, wraparound, double storey and kitchen extensions — designed, approved and built by one team.
Design & Build
Feasibility, design, approvals and construction under one contract, with one person accountable for the result.
Loft Conversions
Dormer, hip-to-gable, L-shaped and mansard conversions that add a bedroom and bathroom without touching the garden.
Architectural Design & Planning
Feasibility, concept design, planning applications and Building Regulations drawings, priced with construction in mind.
Discuss your project
Discuss a double storey extensions project
Send a few details about the property, what you want to achieve and your approximate budget. Stoyan reads every enquiry and replies within one working day.
What happens next: a short call, a site visit if it is a good fit, then a written proposal. No charge, no obligation.