A good extension feels as though it was always part of the house. It is bright, warm, properly signed off under Building Regulations and built by a firm that told you the price, the timescale and the plan before the first brick was laid. The builder matters as much as the design, because most extension regrets come from how the job was run rather than how it looks. Below we cover what to look for in the extension itself, what it should cost, and how to vet a builder, with the checks we would want you to make of us.
TL;DR
- A good extension suits how you live, meets Building Regulations, is well insulated and flooded with natural light, and matches your home’s character.
- Budget roughly £1,800 to £3,000 per m² for a single-storey build, plus a contingency for surprises.
- Get at least three itemised written quotes and compare the approach, not just the bottom line.
- Ask for public liability insurance, references, a written contract, staged payments and an insurance-backed guarantee.
- Check planning and Building Regulations early. They are two separate things and you need to know about both.
Table of Contents
- What makes a good extension
- Which type of extension suits your house?
- What an extension costs
- Planning permission and Building Regulations
- How to choose the right builder
- Perspective: what we have learned in 35 years
- Common mistakes to avoid
- Working with us
- FAQ
- Sources
What makes a good extension
Before you speak to anyone, it helps to know what “good” means. We think of it as six things.
It solves a real problem
The best extensions start with a clear brief. Are you short of a family kitchen, a downstairs bedroom, a home office or somewhere for teenagers to escape to? Experts at Which? Trusted Traders point out that homeowners often rush this stage. Deciding whether speed, cost or minimal disruption matters most lets a builder tailor the approach and quote far more accurately.
A useful exercise: write down the three things the new space must do, and the one thing you would happily give up. That list will save you from adding features that look good on paper but never get used.
It flows from the existing house
An extension that is bolted on awkwardly, with a step down, a dark corridor or a kitchen that no longer connects with the garden, will annoy you daily. Good design considers how the old and new rooms connect, where the light comes from and how you will move through the space. Our guide to extension design ideas to maximise space and light goes deeper on this.
It is built to stay warm and dry
Modern glazing, proper insulation and well-detailed junctions between old and new walls make the difference between a room you use all year and one you avoid in January. Energy performance is part of Building Regulations, which is one reason compliance is more than paperwork.
It uses materials that match the job
Cheap materials are a false economy. Matching brick, a decent roof system and quality windows and doors affect how the extension looks in ten years and how much it costs to heat. Bi-folding doors, aluminium patio doors and French doors can all work well, but the frame, the glass specification and the installation matter as much as the style.
It has the right structure
Many of the best open-plan spaces involve removing or opening up a wall. That calls for calculations, a correctly sized steel beam (often called an RSJ) and proper support while the work is done. A structural engineer’s design is what your building control officer will want to see, and what a structural engineer’s fees and timelines look like is worth understanding early, even though that guide is written with London in mind and prices in the North West can differ.
It adds value you can feel
Some extensions add more value than others, and the best ones add space that works. If resale value is a factor, read our guide to home extensions that add value before you commit to a layout.
Which type of extension suits your house?
The right shape depends on your plot, your budget and your goals.
| Type | Best for | Things to weigh up |
|---|---|---|
| Single-storey rear | Kitchen-diners, family rooms | Rear garden depth, light to existing rooms |
| Wraparound | Semis and detached homes needing a big open-plan space | Structural work at the corner, planning limits |
| Double-storey | Bedrooms plus a ground-floor room | Usually more likely to need planning permission |
| Garage conversion | Playroom, office, extra bedroom | Floor level, insulation, Building Regulations |
| Orangery or garden room | A bright, year-round room | Roof design, heating, glazing |
| Conservatory roof replacement | Rooms too hot or cold to use | Existing frame must be sound |
If you are weighing options, our pages on house extensions, kitchen extensions and garage conversions show what we build across Warrington, St Helens, Wigan and the surrounding towns.
What an extension costs
Costs vary with size, finish, ground conditions and glazing. One 2026 UK guide puts single-storey extensions at between £30,000 and £70,000, or around £1,800 to £3,000 per m², with basic finishes at the lower end and high-end finishes at the upper end. Treat those as a guide only. A kitchen extension with bi-folding doors, underfloor heating and a new kitchen will land higher than a simple bedroom addition.
Here is how we suggest you plan the budget:
- Work out the build cost from at least three detailed quotes.
- Add design, structural engineer’s calculations and Building Regulations fees.
- Add a contingency of around 10 to 15 per cent for the unexpected, such as poor ground or hidden drains. That is our own rule of thumb rather than an official figure.
- Allow for the fitted kitchen, flooring, decoration and any garden reinstatement if they are not in the quote.
We do not publish set prices because every home is different, but we give a fixed-price written quotation once we have seen your property and agreed the specification with you.
Planning permission and Building Regulations
These are two different systems, and people often mix them up.
Planning permission. Many extensions in England come under permitted development, so no planning application is needed if you meet size, height and position limits. According to the Planning Portal, single-storey rear extensions are generally limited to 4m for detached houses and 3m for other houses, with a maximum height of 4m. Those rights do not apply to flats or maisonettes, and are tighter in Conservation Areas. Larger rear extensions may need prior approval from the council. Always check with your local planning authority, whether that is Warrington, St Helens, Wigan or Knowsley. Our guide to permitted development for house extensions explains the detail.
Building Regulations. This covers structural safety, fire protection, drainage, insulation and ventilation. The Planning Portal states that most extensions need Building Regulations approval, and you apply to a building control body, either your council or a private approved inspector. Applications can go in as full plans, which has a five-week decision target, or as a building notice for smaller jobs. GOV.UK explains who needs to apply: if the work is done by someone registered with a competent person scheme, they can self-certify certain work, such as window installation.
A builder who says you do not need Building Regulations on a normal extension is a red flag. Skipping them can cause problems when you sell, because a buyer’s solicitor will ask for the completion certificate.
For garage conversions in particular, see what building regulations apply to garage conversions.
How to choose the right builder
Choosing a builder is the biggest decision in the project. Here is the process we would follow if we were hiring.
1. Find builders properly
Which? Trusted Traders suggests that rather than relying only on friends’ recommendations, the best approach may be to find people in your area who have had similar work done and, if possible, go and look at the finished job. In places like Great Sankey, Penketh, Culcheth or Haydock, a neighbour’s extension two streets away is the best reference there is.
Look at trade body membership too. We are members of the Guild of Master Craftsmen, and we are an approved HUP! Building System partner, an approved UltraRoof installer and an authorised REHAU installer. Those details are worth asking any builder about: which bodies they belong to, and what that means in practice. TrustMark is the government-endorsed scheme, and its Customer Charter sets out the standards a registered business signs up to, including clear communication and a tidy, safe workplace.
2. Get at least three itemised quotes
Compare like with like. Quotes are about more than price: you are also comparing each trader’s approach, estimated time and proposed materials. Be wary of the cheapest, especially where the quote is a single line with no breakdown.
A good quote should list:
- The scope of work, including groundworks, structure, roof, glazing, electrics, plumbing and plastering.
- Who applies for Building Regulations and any planning approval.
- Materials and specifications, with named products where possible.
- Payment stages.
- Start date and expected duration.
- Who removes waste and who deals with subcontractors.
3. Check insurance and credentials
Ask for evidence of public liability insurance and references from previous customers. Citizens Advice lists six steps for getting work done, including checking permissions, finding good traders and getting quotes first. It is also worth reading a broader checklist of contractor background checks before you shortlist.
One question people rarely ask is whether the guarantee is insurance-backed. A normal trader warranty is only as good as the trader. If a firm closes, an insurance-backed guarantee means someone else can still put defects right.
Finally, ask about health and safety. For domestic work, a professional should assess risks and be able to explain how they will keep your family safe on site. If they look blank, that tells you something.
4. Insist on a written contract
A contract exists the moment an offer is accepted, even if nothing is written down, but verbal agreements are the classic source of disputes. Get it in writing: scope, price, stages, timescale, variations and what happens if something changes. Our page on the checklist for choosing extension builders gives you a printable list to work through.
5. Agree sensible payments
Be suspicious of a large upfront deposit. A better approach is staged payments tied to completed milestones, such as foundations, wall plate and watertight, with the final payment held back until you are satisfied. Which? Trusted Traders recommends this approach too.
Perspective: what we have learned in 35 years
We are a family-run firm with over 35 years behind us, and the problems that cause most grief are rarely about bricks. They are about two things: surprises and silence.
Surprises come from rushed briefs and vague quotes. When we quote, we want to know how you live, what you want from the space, and what the existing house is hiding. A fixed-price quotation only works if everything is agreed in advance, so we spend time on the detail up front and walk you through every line.
Silence is the other trouble. Homeowners tell us the worst thing a builder can do is vanish for a week. We would rather you hear from us every day than wonder where we are. We manage the job from design to completion, so there is one team and one point of contact. That is what a fully managed service should mean.
If you remember one thing: choose the builder you trust to tell you bad news early. Every project meets a surprise at some point, whether that is a drain in the wrong place or an old lintel that is not up to the job. How your builder handles it is the real test.
— Gareth
Common mistakes to avoid
- Choosing on price alone. The lowest quote can hide missing items that arrive later as extras.
- Skipping the engineer. Wall removals and RSJs need calculations, not guesswork.
- Forgetting the contingency. Even well-planned jobs meet the unexpected.
- Ignoring the timescale. Ask for a programme. Many single-storey extensions take several months from start to finish, so confirm what each stage includes and when you can expect to use the room.
- Underestimating disruption. Plan where you will cook, wash and work. Our guide to living through a renovation has practical tips.
- Changing your mind mid-build. Variations cost money and time. Settle the layout and fittings before the digger arrives.
Working with us
If you are in Warrington, St Helens, Wigan, Rainhill, Lowton, Culcheth or nearby, we design and build extensions, garage conversions, structural alterations, conservatory roof replacements, orangeries, garden rooms and replacement windows and doors. You get a transparent fixed-price quotation, a clear plan and a team that keeps you informed. See our house extension services, or get in touch for a no-pressure conversation about your project. If you are still at the idea stage, our practical guide to planning a house extension is a good start.
FAQ
How do I know if a builder is reputable?
Check for public liability insurance, references and photographs of finished work, and ask whether they belong to a recognised trade body or TrustMark. Visit a past customer if you can. A reputable builder will happily give you a written contract and itemised quote.
Is it worth getting an architect for an extension?
For complex or two-storey projects, an architect or designer can improve the layout and light considerably. For simpler projects, a design-and-build firm can produce drawings and manage Building Regulations as part of the service. Either way, make sure someone is responsible for the structural design.
How big can an extension be before planning permission is needed?
Under permitted development, single-storey rear extensions are generally limited to 4m deep on a detached house and 3m on other houses, up to 4m high, though larger ones can go through prior approval. Rules are stricter in Conservation Areas and do not apply to flats. Confirm with your local council before you build.
Do I need Building Regulations approval for an extension?
In most cases, yes. It is separate from planning permission and covers structure, fire safety, drainage and energy efficiency. Your builder or designer should arrange this, and you should receive a completion certificate at the end.
How many quotes should I get?
Aim for at least three detailed, written quotes. Compare the scope, materials and approach, not just the total, and ask each builder to explain anything that differs.
Sources
- Before you get work done on your home, Citizens Advice
- How to work with your trader, Which? Trusted Traders
- How to hire guide: builders, Which? Trusted Traders
- TrustMark Customer Charter
- Planning permission and building regulations for extensions, Planning Portal
- Is building regulations approval needed for an extension? Planning Portal
- Building regulations approval, GOV.UK
- Single storey extension cost UK 2026
