1. Your budget needs to be realistic from day one
The single biggest cause of project stress is an unrealistic budget. In 2026, a single-storey rear extension in London or the South-East typically costs GBP 2,800-GBP 3,800 per square metre for mid-specification work. A standard 20 m² kitchen extension therefore sits somewhere between GBP 56,000 and GBP 76,000 before you add professional fees, planning costs or any of the finishes you actually want.
On top of the build cost, budget for architectural drawings (GBP 2,500-GBP 5,000), structural engineering (GBP 900-GBP 1,800), planning fees (from GBP 258), Building Regulations charges (GBP 600-GBP 1,200), party wall agreements (GBP 900-GBP 1,500 per neighbour) and a 10% contingency. For a GBP 70,000 build, that adds roughly GBP 10,000-GBP 16,000 of additional costs that many homeowners forget to include.
At RCB Design & Build we present clients with a full all-in project budget at the review stage - not just the headline construction figure - so there are no surprises once work begins.
2. Planning permission is not always required - but check first
Many single-storey rear extensions in the UK fall within permitted development rights, meaning you do not need formal planning permission. However, permitted development comes with strict size limits, height restrictions, material requirements and boundary setbacks. Get any of those wrong and the work is technically unlawful, which becomes a serious problem when you sell.
If your property is in a conservation area, subject to an Article 4 direction, is a listed building, or is a flat or maisonette, permitted development rights are typically restricted or removed entirely. The first thing we do on any project is check the council's planning constraints map - it takes 30 seconds and prevents weeks of wasted design time.
Even when planning permission is not required, we strongly recommend applying for a Lawful Development Certificate (LDC). At around GBP 129, it gives you written council confirmation that your project is lawful - invaluable when you eventually sell.
3. The Party Wall Act may apply to your project
If your extension involves excavating foundations within three metres of a neighbouring building, building on or near the boundary line, or cutting into a shared wall, the Party Wall etc. Act 1996 almost certainly applies. This is a separate legal process from planning permission and is frequently overlooked by homeowners.
You must serve formal notice on each affected neighbour at least two months before work starts for new wall construction, or one month for excavation works. If your neighbour dissents - or simply does not respond within 14 days - a party wall surveyor must be appointed to produce an award detailing the works and recording the condition of the neighbouring property.
Party wall costs typically run GBP 900-GBP 1,500 per neighbour when a surveyor is needed. Factor this into your budget and your timeline from the start. We handle party wall coordination as part of our pre-construction service so nothing falls through the cracks.
4. Building Regulations approval is non-negotiable
Planning permission deals with how a building looks from outside. Building Regulations deal with how it is actually built - structural safety, fire safety, insulation, drainage, ventilation, electrical safety and energy efficiency. Every extension in England and Wales requires Building Regulations approval, whether or not it needs planning permission.
You can apply through your local council's Building Control department or use an approved private inspector. Either way, an inspector will visit at key stages during construction - foundations, damp-proof course, drainage, structural steelwork, insulation and completion - and issue a completion certificate at the end.
Never accept a builder who suggests skipping Building Regulations. Without a completion certificate, your extension is technically non-compliant, your home insurance may be void for related claims, and you will face serious problems when selling. As FMB and TrustMark accredited contractors, RCB treats Building Regulations compliance as the absolute baseline, not an optional extra.
5. Choosing the right builder matters more than the price
The cheapest quote is almost never the best value. When one quote is significantly lower than others, scope has almost always been stripped out - typically structural design, proper insulation, fire-rated doors, or genuine Building Regulations sign-off. We see those projects on rescue jobs regularly.
Look for builders who hold Federation of Master Builders (FMB) membership, TrustMark Government Endorsement and verified Checkatrade reviews. Check their insurance certificates - public liability of at least GBP 2 million, employer's liability of GBP 10 million, and contract works insurance for the project value. Ask to visit a current or recently completed site.
Use a proper written contract such as a JCT Minor Works or JCT Homeowner Contract. Any builder who resists putting things in writing is telling you everything you need to know.
6. Timelines are longer than you think
A typical single-storey rear extension takes 12-16 weeks on site. But the pre-construction phase - design, planning, structural engineering, Building Regulations submission, party wall notices and procurement - adds another 8-16 weeks before a spade hits the ground. From first phone call to moving back in, six to nine months is realistic for most extension projects.
Weather, material lead times and specialist availability can all stretch the programme further. Structural steelwork and bespoke glazing units commonly have 4-6 week lead times that need to be factored into the procurement schedule well before site work starts.
We provide clients with a detailed programme at contract stage showing every phase, every milestone and every lead-time item, so expectations are set properly from the outset.
7. Living arrangements during the build need planning
Living in a house while it is being extended is possible but requires preparation. A rear extension means losing your kitchen for weeks - sometimes months. Dust, noise, tradespeople arriving at 7:30am and restricted access to your garden are all realities of site life.
Consider setting up a temporary kitchen in another room with a microwave, kettle and portable hob. If you have young children, elderly family members or work from home, think honestly about whether staying is practical. Some clients move out for the structural phase and return for the finishing stages.
We discuss living arrangements with every client at the project review and plan the works sequence to minimise disruption - for example, maintaining water and heating for as long as possible before the switch-over to new services.
8. Good design is worth paying for
An architect or architectural designer does more than draw lines on paper. Good design maximises natural light, creates efficient circulation, considers sight lines from key living spaces, and ensures the extension relates properly to the existing house and the garden beyond it.
Spending GBP 3,000-GBP 5,000 on proper architectural design can save tens of thousands by avoiding poor layouts that need changing on site. It also produces accurate drawings that allow builders to quote like-for-like, structural engineers to design efficiently, and Building Control to approve without delays.
We work with a trusted network of architects and designers who understand construction realities - their designs are buildable, not just beautiful on screen.
9. Structural engineering is not optional
Any extension that involves removing or altering load-bearing walls, adding steelwork, or changing the roof structure requires structural engineering calculations. These calculations are a Building Regulations requirement - the structural engineer specifies the size of steels, the type of foundations, and the connection details that keep the building standing.
Structural engineer fees for a standard extension typically run GBP 900-GBP 1,800. This is not an area to economise. Under-specified steels mean Building Control rejection and costly redesign. Over-specified steels waste money on unnecessary materials and crane hire.
We commission structural design early in the pre-construction phase so that steel and foundation specifications are locked before work begins - no surprises, no delays, no expensive changes mid-build.
10. Warranties and sign-off protect your investment
When the extension is complete, you need three things: a Building Regulations completion certificate from your inspector, an electrical installation certificate from your electrician, and a gas safety certificate if any gas work was carried out. Without these, your home insurance and future sale are both at risk.
Beyond statutory certificates, look for builders who offer a meaningful defects liability period - typically 12 months - during which any snagging or defects are rectified at no additional cost. For structural work, a 10-year structural warranty (such as those underwritten by the FMB or specialist warranty providers) adds a further layer of protection.
At RCB Design & Build, every project is handed over with a full documentation pack including all certificates, warranties, an operations and maintenance manual, and a clear point of contact for any post-completion matters. Our TrustMark registration means your work is also backed by the government-endorsed TrustMark framework.
Frequently asked questions
How much does a home extension cost in London in 2026?
A single-storey rear extension in London typically costs GBP 2,800-GBP 3,800 per square metre in 2026. A standard 20 m² kitchen extension therefore sits between GBP 56,000 and GBP 76,000 for the build alone, with an additional GBP 10,000-GBP 16,000 for professional fees, planning, Building Regulations and contingency.
Do I always need planning permission for an extension?
No. Many single-storey rear extensions fall within permitted development rights and do not require formal planning permission. However, properties in conservation areas, Article 4 zones, listed buildings and flats typically need full planning consent. Always check before designing.
How long does it take to build a house extension?
A typical single-storey rear extension takes 12-16 weeks on site. Add 8-16 weeks for the pre-construction phase covering design, planning, structural engineering and procurement. From first contact to completion, six to nine months is realistic.
What is the Party Wall Act and does it affect my extension?
The Party Wall etc. Act 1996 applies when you excavate foundations within three metres of a neighbouring building, build on or near a boundary, or cut into a shared wall. You must serve formal notice on affected neighbours before starting work, and a surveyor may need to produce a party wall award.
Should I move out during a home extension?
It depends on the scope and your household. Living in during a rear extension is common but means losing your kitchen temporarily, dealing with dust and noise, and having tradespeople on site from 7:30am. Families with young children or people working from home should consider moving out for the structural phase at minimum.
Planning a project of your own?
Book a free project review with the RCB team. We will respond within one working day.