North London Rear Extension Cost Calculator
Estimate the cost of a single or two storey rear extension in North London / Greater London. This calculator gives an indicative budget range based on typical assumptions — useful for early-stage budgeting before you commission surveys, drawings or builder quotes.
It’s designed for homeowners across Barnet, Enfield, Haringey, Camden, Islington and Hackney who want a sensible starting point and a clearer idea of what’s included and excluded.
Costs vary with structure, drainage, access and specification, so treat this as a guide rather than a fixed price.
Use the calculator below to estimate costs for common rear extension types.
If you want a quick sense-check for your address, you can book a free 15-minute Planning & Budget Check after you’ve used the calculator.
Estimate Your Extension Cost
Estimated Construction Cost excl. VAT
£0
*Indicative estimate based on Steadfox 2025 rates. Subject to site conditions and specification.
What this calculator is (and what it isn’t)
This tool gives an early budget range for common rear extension scenarios in Greater London, based on typical assumptions. It’s useful for deciding:
whether your idea is broadly affordable
whether you need to reduce size / change specification
what you should ask builders to price when you’re ready
It isn’t a substitute for a measured design and builder quotes. Two extensions with the same footprint can cost very different amounts depending on structural alterations, drainage and finishes.
Why North London extension costs vary so much
Online prices swing wildly because people compare different scopes. In practice, the biggest cost drivers tend to be:
Structural openings and steelwork
Large openings to the rear, removing load-bearing walls, and supporting existing floors often add meaningful cost.
Groundworks and drainage
Manholes, drain runs, soakaways and ground conditions can turn a “simple” extension into a more complex job.
Access and logistics
Restricted side access, no driveway, party walls, skip permits and working hours all matter in London.
Specification and finishes
Glazing type, insulation approach, underfloor heating, kitchens and flooring are where budgets often drift.
What most people forget to allow for
Even when the build cost looks fine, projects go off-track when “extras” are discovered late. Depending on your situation, you may need to budget for:
VAT (on construction, and often on professional fees)
professional fees (architect, engineer, SAP assessor, party wall surveyor)
statutory fees (planning, building control)
party wall matters (if you’re on/near a boundary)
utilities/drainage alterations
a contingency for unknowns, especially early on
The calculator helps with a baseline. The next step is making sure you’re not comparing apples with oranges.
Planning route in North London: PD vs planning permission
Many houses can extend under Permitted Development, but not always. Common reasons you may need planning permission include:
conservation areas / additional restrictions
previous extensions or removed PD rights
flats/maisonettes (different rules)
proposals that exceed PD limits on depth/height/materials
If you’re unsure, don’t guess, it’s one of the easiest ways to waste time and fees.
Will my extension likely be approved — and stay within budget?
Book a free 15-minute Planning & Budget Reality Check with an ARB-registered architect for your address.
You’ll leave with:
Planning route PD vs. Planning Permission
Budget range - what’s included/ excluded
Next steps and in what order
No obligation. No hard sell. Practical advice only.
Please have your postcode and a photo or Rightmove link ready.
FAQs
-
It’s accurate enough to set expectations and avoid obvious budget mistakes — as long as you treat it as an indicative range and understand what’s included. It won’t replace a measured design and a builder’s price.
-
VAT and fees depend on scope and the procurement route. The key is to be clear on what’s included and excluded when comparing figures.
We show VAT separately from the construction cost so you can benchmark the construction figure more easily against contractors’ estimates, as builders’ quotes often exclude VAT.
Professional fee estimates and statutory fees already include VAT
-
No. It’s usually more than a single storey, but it isn’t a simple multiple. Structure, openings and fit-out drive the difference and larger extensions can sometimes work out slightly cheaper per m² because some costs don’t scale linearly (setup, prelims, scaffolding).
-
Yes, as an early benchmark. The biggest swings tend to come from site constraints and specification rather than the postcode alone.