How the numbers are worked out

A report covers one house and one job, in these sections, as in the sample report.

• The headline: the likely total, its range and a summary.
• Your property: what the public records say.
• What it will cost: build, extras, fees and contingency.
• Schedule of works: each item with quantity and rate.
• Planning: the route for this house: each rule with its result.
• What else applies: party wall, building regulations, drains.
• Timeline: design, approvals and build, in weeks.
• What moves the price: the job re-priced with one change.
• What else this house could take: other jobs that fit.
• Risks for this job: what could push cost or time.
• Questions to ask anyone who quotes: for like-for-like quotes.
• Next steps: what to do first.

A paid report also lets you change the size and finish and see the figures move.

Three public sources are read for each address. If one fails, the report is still written from your answers, and says what was checked.

postcodes.io

Turns your postcode into a borough, which sets the local build rate, and a map point, used to read the planning register.

The EPC register

The energy certificate gives house type, floor area, age band and past extensions. We match it on the address you typed. The register's own address lines are licensed for energy purposes only, so we never show them and never store them.

planning.data.gov.uk

Conservation areas, Article 4 directions, flood zones and nearby listed buildings.

Where the data stops

• The point on the map is the centre of the postcode, not your plot. A house near the edge of a conservation area can be read wrongly either way.
• Coverage is partial. Not every London borough has published its conservation areas, Article 4 directions or listed building outlines to the national register.
• So “none found in open data” means exactly that. It never means the house is not designated. Check with the council before relying on it.
• An energy certificate can be years old, and some houses have none. Where the record is missing, the report uses your answers.

Attribution

Contains OS data © Crown copyright and database right 2026. Contains Royal Mail data © Royal Mail copyright and database right 2026. Contains National Statistics data © Crown copyright and database right 2026. Contains public sector information licensed under the Open Government Licence v3.0 (planning.data.gov.uk; Energy Performance of Buildings data, MHCLG).

The engine is a set of rules in code: the same answers give the same figures every time. It prices at current London rates, reviewed and versioned, with the date shown on every page.

Rates checked 20 October 2026. Statutory fees from 1 April 2026. All figures include VAT.

Schedule of works

The build is priced from the bottom up. The size of the job becomes quantities (floor, walls, roof, rooflights, steels), each multiplied by a unit rate. If you give no size, a typical one is assumed and the report says so. The typical rear extension here is assumed 3.5 m deep by 5 m wide, about 17.5 m², and its build comes to £49,000 to £82,000.

Rates

Supply and fit, including VAT. Every report prints the rates version it was priced on.

Borough

A multiplier on the build rate, for parking, access and labour. Most boroughs sit at the London baseline; the dearest inner boroughs run up to 15% above it.

Finish

You choose one of three finishes: straightforward, good quality, high end. Finishes follow your choice in full; structure only in part.

Extras

Kitchen, bathroom, bifold doors and knocking through are separate lines outside the schedule, so nothing is counted twice.

Fees

Design is a share of the build, with a minimum. Council planning fees are the national statutory figures, from 1 April 2026: a householder application £548, prior approval for a larger home extension £249, and a lawful development certificate £274. Building control, party wall surveyors and sewer build-over agreements are ranges.

Contingency

The report adds a contingency to hold back, as a share of the build: 10% for straightforward, 12% for good quality and 15% for high end. It is money to have available, not a certain cost.

VAT

All figures include VAT at 20%. Builders often quote without VAT, so add 20% to any quote that says 'plus VAT' before comparing it with this report.

For the typical rear extension, the total to have available comes to £61,000 to £114,000, before any extras.

The screening tests your job against the householder permitted development rules: Schedule 2, Part 1 of the General Permitted Development Order 2015.

Class A

Rear and side extensions: depth, height, eaves, width, boundaries, garden cover.

Class B

Roof additions such as dormers: volume, height, slope, set-back.

Class C

Other roof alterations such as rooflights: projection and height.

Each rule is a separate check. Each check reports pass, fail or check, and names the paragraph of the order it comes from. A fail moves the job onto a different route. A check means the answer turns on something we cannot see, such as the depth measured from the original rear wall. Flats have no permitted development rights. A listed building needs listed building consent, and an Article 4 direction can remove the rights.

A single-storey rear extension deeper than the standard limit can still go ahead under the larger home extension scheme. The council consults the neighbours and decides whether to approve the effect on them. This is prior approval, lighter than a full application. The typical rear extension on this page screens as “Larger home extension: prior approval needed”.

Where a job passes every check, it is normally permitted development, subject to the limits checked in the report. The report then advises a lawful development certificate, which is written proof that the work was lawful.

What the screening is not

The screening reads the published rules against your answers. It is not a planning decision and it is not planning advice. The council decides. Where a constraint is found, or the register has no data for the area, the report says to check with the council.

The words are written by Claude, a language model made by Anthropic. It is handed the finished figures as text, already rounded and formatted, with the facts about the house. It is told to quote each figure exactly as given and never to change, add or recompute one. It writes the summary, the notes on the property, the costs and the planning, the risks, the questions and the next steps.

Its answer comes back as structured output: named fields of text that the page places around the tables. The tables and the figures in them are drawn from the engine, not from the writing.

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.

Not useful? Reply to your report email within 14 days and we refund it, no questions asked.

How close are the numbers?

They are ranges on purpose. Where your job lands depends on the ground, the state of the house and your choices of kitchen and glazing, which no desk estimate can see. The report shows what moves the price, so you can see which choices matter most.

Why does my builder's quote differ from the report?

A quote is priced by someone who has seen the house, at their own rates and margin. Often the quote is shown without VAT, leaves out the fees or the contingency, or excludes items the report includes. Compare it line by line with the schedule of works, and add VAT to any quote that says plus VAT.

Do you check my actual plot?

No. Nobody visits the house. The planning register is read at the centre of your postcode, not at your plot boundary, and the house details come from its energy certificate and your answers. Garden size, side access, trees and drains need checking on site, and the report lists them as questions to ask.