Guides to extension and loft costs and planning in London
Each guide answers one question with figures from the same engine as the report, for a typical London house. Your own house will differ: the free check and the report price it.
Rates checked 20 October 2026. Statutory fees from 1 April 2026. All figures include VAT.
- How much does a loft conversion cost in London in 2026?
What a rooflight, rear dormer, L-shaped, hip-to-gable or mansard loft conversion costs in London, all in and per square metre, by finish and by borough, at current London rates.
- How much does a rear extension cost in London in 2026?
What a single-storey or double-storey rear extension costs in London, all in and per square metre, by finish and by borough, with what the steel, the glazing and the fees add, at current London rates.
- Do I need planning permission for a rear extension?
When a rear extension is permitted development, when it needs prior approval, and when it needs a planning application: the depth, height and garden limits by house type, with the GPDO paragraph for each and the current council fees.
- Do I need planning permission for a loft conversion?
When a London loft conversion is permitted development and when it needs a planning application: the Class B volume allowance, dormer rules, hip-to-gable, mansards, conservation areas, Article 4, flats and the lawful development certificate.
- How much does a side-return or wraparound extension cost in London?
What a side-return or wraparound extension costs in London, all in and per square metre, by finish and by borough, with why the rate per m² is high and what the party wall adds, at current London rates.
- What are the planning fees for an extension in 2026?
The statutory planning fees for a house extension or loft conversion in England from 1 April 2026: householder application, prior approval, lawful development certificate, the Planning Portal charge, and building control on top.
- What do an architect, engineer, building control and party wall cost for an extension?
What the designer or architect, structural engineer, building control, council fee and party wall surveyors cost for a London extension or loft conversion, line by line, at current London rates.
- How long does an extension or loft conversion take?
How many weeks a rear extension or loft conversion takes from first sketch to sign-off, phase by phase, and how the planning route, lead times and the ground change the calendar.
- Why are extension quotes so different?
Why two quotes for the same London extension or loft can be tens of thousands of pounds apart: scope, finish, size, VAT, fees, contingency, provisional sums and risk, and how to compare them like for like.
- Can I extend in a conservation area or under an Article 4 direction?
What a London conservation area or an Article 4 direction changes for a rear, side or loft extension: which work stays permitted development, which needs a planning application, how to tell whether a direction applies, and the council fees.
- What contingency should I allow for an extension?
What contingency to allow for a house extension or loft conversion in London: the share of the build to hold back by finish level, the pounds on a typical job, what usually uses it up, and how to hold it.
- Does an extension or loft add value in London?
Whether an extension or loft pays for itself depends on your street, not a formula. What the cost side of the sum looks like in London: all-in totals, cost per square metre, fees, contingency and time, at current London rates.
How the numbers are worked out · Costs by borough
This report is an informed estimate built from public data about your property and current London build costs. It is not a survey, a quote, a valuation or planning advice. Always get written quotes and, where noted, professional advice before committing to work.