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House Extension Planning Checklist for London Homeowners
A house extension is one of the most effective ways to gain space without moving. But it's also one of the most commonly mis-started projects — where homeowners spend money on architects, surveys, and even planning applications before establishing whether their idea is actually viable as they've imagined it.
This checklist is designed to help you think through the key questions before you commit significant time or money to the process.
1. Establish whether you need planning permission
Not all extensions require full planning permission. Under Permitted Development rights, many rear extensions and loft conversions can proceed without applying to the council — as long as they meet specific size limits and design conditions.
However, Permitted Development rights can be restricted or removed entirely in certain circumstances:
If your property is in a Conservation Area, the rights are more limited, particularly for side extensions and alterations visible from the street.
If you live in a flat or maisonette, PD rights generally do not apply.
If the property has been extended before, you may have used up part of the PD allowance.
If an Article 4 Direction is in place in your area (common in parts of London), PD rights may be withdrawn entirely.
It's worth getting a professional pre-application check before assuming PD applies. A Lawful Development Certificate from the council gives you formal written confirmation that your proposed works are lawful — and that document has real value if you ever sell.
2. Check your title deeds for restrictive covenants
Some properties carry restrictive covenants in the title — legal obligations that can prevent certain types of development, even if the council would permit it. If your property has a covenant preventing extensions, building near a boundary, or changing the use of outbuildings, you may need indemnity insurance or formal consent from the beneficiary before proceeding.
Your solicitor can review the title register, or your architect can flag if it's something that needs legal attention.
3. Identify party wall implications early
If your extension involves building at or near the boundary with a neighbour, excavating for new foundations, or working on a shared wall or structure, the Party Wall etc. Act 1996 is likely to apply.
This means serving formal notice on your neighbour before work starts — typically two months in advance. If your neighbour consents in writing, the matter is straightforward. If they dissent, a Party Wall Award must be agreed between Surveyors before works can commence.
Failing to serve notice can result in injunctions, claims for damage, and disputes that halt a live project. Start this process early.
4. Understand your drainage and build-over position
If your extension passes over or near a public sewer, you may need approval from your water authority before building. Thames Water, for example, has a Build Over Agreement process that applies to sewers within a specified distance of your proposed foundations.
This is easy to check using the Thames Water Sewer Map (or your regional water authority equivalent) and is worth doing before design fees are committed.
5. Confirm architectural drawing requirements
Different stages of the project require different levels of drawing. Planning drawings are not the same as construction drawings. You'll need construction-ready technical information — including structural details, thermal specifications, drainage layouts, and connection details — before a contractor can price the job accurately or build safely.
Make sure you understand which drawings are included in your architect's scope and which will be produced later, at what cost.
6. Engage a structural engineer early
Most extensions require structural input — at minimum, a calculation for the new foundations and any structural openings. On older properties, ground conditions, existing load paths, and substructure risks need to be understood before final pricing is possible.
Getting your structural engineer involved alongside the architect, rather than after planning, prevents situations where the approved design can't be built as drawn without expensive redesign.
7. Plan the build sequence before you agree a start date
Extensions often overlap with live occupation — meaning you, your family, and your belongings are in the building while work happens around you. Think through:
Where will you cook if the kitchen is affected?
Will the back of the house be weather-tight during the construction phase?
Are there young children or pets who need to be kept away from site?
Sequencing these questions upfront — and discussing them honestly with your contractor — prevents the misunderstandings that create tension on live projects.
If you've worked through this checklist and are ready to talk through your extension project, we're here.
RCB Design & Build handles design-and-build extensions across London — including pre-construction support, structural coordination, party wall management, and full principal contractor delivery from first meeting to handover.
Call us on +44 7359 872594, email contact@rcbgroup.co.uk, or visit rcbgroup.co.uk.
Kind regards, Vasi J. & rcbGroup team contact@rcbGroup.co.uk www.rcbGroup.co.uk +44 7359 872594 | +44 7448 189592 FMB | TrustMark | Checkatrade | TrustATrader Verified. Insured. Trusted.