the extension handbook
Forty years. 4,500 planning applications. The same five mistakes.
A short bundle of guides for UK homeowners planning an extension between £30,000 and £150,000. The mistakes that cost the most are not the ones you would expect, and almost all of them are made in the first two weeks of a project, before any building work begins.
40 years of UK practice · 4,500 planning applications · 3 guides · 2 checklists · one £1,000 promise
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Who I am, and why this exists
I have spent forty years in UK practice working on extensions, conversions, and renovations across hundreds of projects, with a personal archive of around 4,500 planning applications to draw from. The Extension Handbook is the bundle I built because the same conversations were happening in my office over and over.
A homeowner would arrive with an extension already half-planned, often half-priced by a builder, and within ten minutes of looking at it I could see two or three places where the project was about to cost them more than it should. Not because their builder was dishonest. Builders mostly are not. Because the things that go wrong on extension projects are quiet, technical, and invisible to a homeowner who has not been shown where to look.
The bundle is what I would tell a homeowner if they sat in my office for an hour. The patterns I have seen 4,500 times. Written down.
the extension handbook
Five moments where the money goes
Moment 1
The planning assumption that cost £2,000 (Leamington Spa)
A homeowner in Leamington Spa wanted a modest side extension. Her designer told her it would qualify as Permitted Development. It did not. The property sat in an Article 4 Direction area that removed standard PD rights, and the extension required a full planning application. By the time this was established, she had paid for drawings that needed to be redrawn, and the build was delayed by fourteen weeks while the application was processed.
The Permitted Development rules are not complicated. But they have exceptions that are easy to miss and expensive to discover after the drawings are done. The Planning Guide covers them in full.
Moment 2
The quote that was never a budget (Warwick)
A homeowner in Warwick received a quote of £68,000 for a rear extension. He proceeded to design stage on that basis. When the tender went out to three builders, the lowest came back at £86,000. The original figure had been an early conversation estimate, not a priced scope. The homeowner had spent £3,100 on design fees building a project around a number that was never real.
The first number a builder gives you is not a budget. It is a conversation opener. The Contracts Guide explains what a real budget looks like and how to get one before committing to design fees.
Moment 3
The application that should have been two (Evesham)
A homeowner in Evesham had a large property and a long list of work across several phases. He was advised to submit applications one phase at a time. He did. What nobody told him was that submitting both phases in a single application would have cost the same in fees, taken the same time, and given him a consented scheme for the full scope of works in one go. The second application cost him £1,600 in fees and another eight weeks.
The Planning Guide covers consolidation strategy for phased projects. It is one of a dozen moves that are obvious once you know to look for them.
Moment 4
The validation surprise that cost £1,800 (Stratford-upon-Avon)
A homeowner in Stratford-upon-Avon engaged a designer to draw up plans for a rear extension. The designer submitted the application. The council wrote back requesting a Bat Survey, a Tree Survey, and a Heritage Statement before they would begin assessing it. The reports cost £1,800. The four-week delay lost the homeowner their builder's slot, pushing the build back by two months.
Most local planning authorities publish a validation checklist online. A planning officer will confirm what is required for a specific site over the phone in under two minutes. The Planning Guide covers validation requirements and how to establish them before appointing a designer.
Moment 5
The notice that should have been served first (Coventry)
A homeowner in Coventry began a loft conversion. On day two, a neighbour asked why no Party Wall Notice had been served. The works halted. The statutory notice period under the Party Wall Act is two months if a neighbour dissents. The homeowner had paid for a builder's mobilisation, scaffolding erection, and two days of labour before the project stopped.
The Party Wall Act applies to a significant proportion of residential extension and conversion work. The Planning Guide covers when it applies, what it requires, and how to serve notice correctly before any work begins.
the extension handbook
What's in the bundle
Three guides. Two checklists. One complete picture.
The Planning Guide
Everything you need to know before submitting a planning application. Permitted Development rules and their exceptions, Article 4 Directions, validation requirements, how to read a decision notice, and what to do if the council gets it wrong.
The Contracts Guide
How to read a building contract, what the dangerous clauses look like, and how to negotiate them. Payment schedules, retention, variations, and what happens when a builder walks off site.
The Project Management Guide
How a well-run extension project actually works, week by week. Programme management, stage sign-offs, the questions to ask at every site visit, and how to close a project without a dispute.
Builder Contract Checklist
A one-page checklist for reviewing any building contract before you sign it. Thirty-one clauses, plain-English notes on each.
Variation Order Checklist
A one-page checklist for managing every change to the agreed scope. Use it every time a builder proposes a variation — before you agree to anything.
the extension handbook
Three guides plus both checklists, £67 for the full set
The £1,000 promise
If you read the bundle and do not believe it has been worth at least £1,000 to your project, your money comes back. No claim form, no qualifying conditions, no fine print.
The bundle costs £67. The guarantee is for £1,000. The difference is the confidence that comes from forty years of seeing the same projects go wrong in the same ways.
Why the question is when, not whether
A planning decision in England takes 8 weeks for most household applications, longer in conservation areas. Validation can add 4 weeks if the application pack is incomplete. Most reasonable builders book three to six months ahead. The Party Wall Act has a two-month notice period if the neighbour dissents.
A homeowner who starts thinking about an extension in March and wants to break ground in summer has roughly fourteen weeks to make every decision that determines whether the project goes well. Most of those decisions cannot be undone once they are made.
The bundle is designed to be read in an evening and applied across a project from the first conversation onwards. The earlier it is read, the more decisions it changes. By the time the planning application is submitted, half the value is already gone. By the time the builder is on site, most of it is.
£67 to know what you don't know
The five cases on this page total roughly £11,700 in losses across three UK homeowners. Each loss was preventable. Each homeowner thought they were managing their project sensibly. None of them were wrong about that. They just did not know what the questions were.
40 years of UK practice · 4,500 planning applications · 3 guides · 2 checklists · one £1,000 promise