How to Plan a House Extension: A Step-by-Step Guide
Quick Answer
Planning a house extension involves eight stages: defining your brief, checking your planning position, appointing an architect, managing the party wall process, submitting to building control, getting structural calculations, tendering the build, and managing the construction. From initial idea to start on site takes four to six months for a permitted development extension and six to nine months if full planning permission is required.
Here's what nobody tells you upfront: planning a house extension is a process with a very specific sequence. And when you get that sequence wrong — appointing a builder before you've got planning, or serving party wall notices after you've already started on site — you don't just lose time. You lose money. Sometimes you lose the project entirely.
We've surveyed hundreds of homes across Hertfordshire and North London. We've seen extensions stall for eight months because a drain nobody knew about ran straight through the proposed footprint. We've seen homeowners who got a verbal yes from their neighbour, skipped the formal party wall notice, and ended up in a legal dispute that cost more than the extension itself. And we've seen projects go beautifully — on time, on budget, no surprises — because the planning stage was done properly.
The difference between those two outcomes isn't luck. It's sequence. This guide lays out the eight stages of planning a house extension in the order they should happen, with the questions you should be asking at each stage and the decisions that have the biggest impact on your cost and timeline. It's based on how TCM actually runs the planning process for clients — not a generic overview, but the workflow we use on every project.
One thing to be clear about: this guide covers the planning phase only — from initial idea to start on site. If you want to understand what the build itself involves, the House Extension Timeline guide covers the construction sequence week by week. And if cost is your main question right now, the House Extension Cost guide gives current Hertfordshire figures by extension type and finish level.
The Eight Stages of Planning a House Extension
Click each stage to expand the detail and see the questions to ask at that point.
Indicative Planning Timeline
The table below shows a typical timeline for a single-storey rear extension requiring a Lawful Development Certificate (not full planning permission). A full planning application adds four to eight weeks to the planning phase. Stages overlap — the party wall process, structural calculations, and building control application all run concurrently with the design stage.
| Stage | Typical Timing | Notes |
|---|---|---|
| Brief & research | Weeks 1–3 | Define brief, check PD position, appoint architect |
| Design & drawings | Weeks 3–10 | Architectural drawings produced; structural engineer appointed |
| Party wall notices | Weeks 4–12 | Concurrent with design; 2-month statutory notice period |
| Planning / LDC | Weeks 8–18 | 8–10 weeks from valid submission to decision |
| Building control | Weeks 10–20 | Full Plans: submitted with drawings; Building Notice: on commencement |
| Tender & contract | Weeks 16–22 | 3–6 weeks to receive and evaluate quotes; contract signed |
| Build | Weeks 22–42+ | 8–20 weeks on site depending on extension size and type |
Pre-Build Cost Breakdown
Pre-build costs — everything before the contractor starts on site — are often underestimated. The table below shows typical costs for a standard single-storey extension in Hertfordshire in 2025–26. These are paid directly to the professionals involved and are separate from the build contract. The House Extension Cost guide covers the full build cost breakdown.
| Item | Typical Cost |
|---|---|
| Architectural drawings (planning + building regs) | £2,000–£6,000 |
| Structural engineer calculations | £600–£1,500 |
| Lawful Development Certificate | £206 |
| Full planning application | £258 |
| Building control (Full Plans) | £600–£1,200 |
| Party wall surveyor (if required) | £800–£2,000 |
| Build cost (single-storey rear, standard finish) | £35,000–£65,000 |
| Build cost (double-storey, standard finish) | £55,000–£100,000+ |
| Contingency (10–15%) | £3,500–£15,000 |
From Our Work
How TCM Actually Runs the Planning Stage
When a client comes to us with an extension idea, the first thing we do is a free site survey. Not a sales visit — an actual survey. We check the soil conditions, identify any drainage runs in the garden, confirm the planning position, and look at the existing structure to understand what the extension will connect to. That survey takes about 90 minutes and produces a written summary the client keeps regardless of whether they proceed with us.
The survey answers the questions that most homeowners don't know to ask: what depth will the foundations need to be (on London Clay, the answer is almost always deeper than the homeowner expects), whether there's a drain in the way of the proposed footprint, and whether the planning position is straightforward or needs careful handling.
The most common planning-stage surprise we encounter is an uncharted drain running across the proposed extension footprint. In Hertfordshire's older housing stock — particularly the 1930s semis that dominate Borehamwood, Radlett, and Bushey — the drainage layout often doesn't match what's on the utility plans. Discovering this during groundworks is expensive; discovering it during the site survey costs nothing and allows the design to route around it.
The second most common surprise is the party wall position. Homeowners who've spoken informally to their neighbours about the extension often assume that verbal agreement means the party wall process will be straightforward. It usually is — but the two-month statutory notice period still applies, and it runs from the date the written notice is served, not from the date of the conversation. Starting the party wall process at the design stage rather than after planning approval saves four to eight weeks on the programme.
The planning stage is where the project is won or lost. Getting the sequence right — survey, brief, design, party wall, planning, building control, structural calculations, tender — means the build starts with everything in place rather than catching up.
What Goes Wrong (And How to Avoid It)
These are the four most common planning-stage failures we see. None of them are unusual — and all of them are avoidable.
Common Mistake
Appointing a builder before planning is resolved
The builder prices the job, holds a start date, and then the planning application takes three months longer than expected. The builder moves on to another project. You're back at square one with a four-month gap in your programme.
How to Avoid It
Get your planning position confirmed — either an LDC or a planning approval — before you go out to tender. The only exception is if you're running the tender process concurrently with a planning application that's already submitted and progressing normally.
Common Mistake
Skipping the Lawful Development Certificate
The extension is built. Ten years later, you try to sell. The buyer's solicitor asks for the LDC. You don't have one. You need either a retrospective LDC (which requires the council to assess the extension against the PD rules as they were at the time of construction) or indemnity insurance. Both are expensive and slow down the sale.
How to Avoid It
Apply for the LDC before you start. It costs £206 and takes eight weeks. It's the cheapest insurance you'll ever buy.
Common Mistake
Treating the party wall process as an afterthought
The planning approval comes through. You're ready to start on site in six weeks. Then you realise you haven't served party wall notices. The two-month statutory notice period means you can't start for at least eight weeks. Your contractor's programme slips into winter.
How to Avoid It
Serve party wall notices as soon as the design is far enough advanced to describe the proposed work. Run the party wall process concurrently with the planning application, not after it.
Common Mistake
Choosing a builder on price alone
The lowest quote wins. The builder starts on site, hits an unforeseen ground condition, and immediately asks for a variation. The variation isn't priced in the contract. The dispute about who pays for it runs for three months and delays the project by six weeks.
How to Avoid It
Compare quotes on a like-for-like specification, not just a total price. A quote that doesn't include a specification breakdown isn't a quote — it's a number. Ask every builder to price the same drawings and specification, and compare the line items, not just the totals.
Frequently Asked Questions
Related Reading
The 12 Most Common House Extension Mistakes
Knowing the planning stages is one side of the picture. Knowing what goes wrong — and why — is the other. The House Extension Mistakes guide covers the 12 most common errors across planning, contractor selection, and construction, with real examples from TCM's remediation work in Hertfordshire.
Read the guideRelated Services & Guides
House Extension Services
Full-service extensions across Hertfordshire and North London
House Extension Cost Guide
What a house extension costs in Hertfordshire in 2026
Extension Planning Permission Guide
When you need planning permission and how to apply
Permitted Development Rights
What you can build without a planning application
Party Wall Agreement Guide
When notices must be served and what happens if a neighbour dissents
Building Regulations Guide
The inspection schedule and what building control checks at each stage
House Extension Mistakes to Avoid
The 12 most common errors and how to sidestep them
How to Choose a Builder
A checklist for evaluating contractors and their contracts
Start With a Free Site Survey
TCM's free site survey covers planning position, soil conditions, drainage, party wall, and a written summary — before any commitment is made. It's the first step in the planning process and the one that prevents the most expensive surprises.
Or read the House Extension Services page to see the full range of extension types TCM delivers across Hertfordshire and North London.