A straight-talking guide from Excel Construction no ballpark figures, no surprises.
If you have searched for extension costs online, you have probably found a range so wide it is almost useless. £30,000 to £150,000. £1,500 to £3,000 per square metres. These figures are not wrong they are just incomplete. They do not tell you what you will actually pay for a rear single-storey extension in South London in 2026.
We are going to fix that.
Excel Construction has been delivering extensions across South London, Croydon, Surrey, and surrounding areas for over 20 years. We work on high-specification residential projects and act as main contractor on builds featured in the Architects Journal, Homes & Gardens, Archello and Dezeen. We know what things cost. And we believe that if you give people honest numbers, they make better decisions and they tend to choose contractors they trust.
So here is what you actually need to know.
The Honest Answer: £100,000 to £150,000
For a rear single-storey extension in South London in 2026, a well-specified, properly managed project delivered by a reputable main contractor will typically cost between £100,000 and £150,000.
That range assumes:
A typical footprint of 20 to 35 square metres
Standard residential specification quality finishes, not premium bespoke
A knowledgeable architect already engaged or to be appointed
No unusual structural complications such as basements, significant party wall works, or contaminated ground
Full project management by a single main contractor
If your budget is below £80,000, a rear extension of meaningful size in South London is very difficult to deliver well. If someone quotes you £60,000 for a 30 square metre rear extension with a full kitchen refit, ask them exactly what is included and what is not.
The cheapest quote is rarely the cheapest outcome. We have been called in to fix more unfinished or substandard extensions than we can count. The additional cost to rectify poor work is always higher than the saving made on the original quote.
We recently completed a basement conversion in Wandsworth as the third contractor appointed on the project. The previous two had walked away. The client had lost confidence entirely. We restored what had been lost and delivered a result the family are proud of. Twenty years of experience means we have seen most things and we know how to fix them.
What Drives the Cost?
Extensions are priced on the basis of what they include, not just their size. Here are the main cost drivers you need to understand.
1. Structure and Groundworks
This is the foundation of your project literally. Costs vary depending on soil conditions, proximity to neighbouring structures, whether party wall agreements are required, and the structural engineer’s specification. In South London, clay subsoils are common and can require deeper foundations than you might expect.
On a recent project in Tooting, we discovered the public sewer ran directly through the intended foundation line. We redesigned the foundation on site and kept the project on programme.
Typical groundworks and structure: £18,000 to £30,000.
2. Roof Specification
A flat roof with a quality membrane system, adequate insulation, and drainage is the most common choice for a single-storey rear extension. Pitched roofs cost more but may suit your property better. Lantern lights and rooflight glazing add significant cost but transform the light quality of the space.
We are approved installers for the Danosa felt system and approved by Prefa for aluminium and zinc roofing. We will tell you which system is right for your project and which to avoid. The same applies to rooflights: we have specified and installed enough of them to know which suppliers to trust and which to walk away from.
Roof works: £8,000 to £20,000 depending on specification and glazing.
3. External Walls and Openings
The rear wall of the existing house will typically be partially or fully removed to create the open-plan connection most clients want. This involves structural steelwork, temporary support, making good, and new external walling. Brick matching a common requirement in conservation areas across South London adds cost.
Structural openings and external works: £12,000 to £22,000.
4. Internal Fit-Out
Flooring, plaster, paint, joinery, kitchen installation (if applicable), and decoration. This is where specification decisions have the most impact on final cost. A basic fit-out differs significantly from a high-specification finish with bespoke joinery and premium materials.
Most extension projects we deliver connect directly to the dining room or living room, and we frequently add a new cloakroom as part of the same build.
Internal fit-out: £20,000 to £40,000 depending on specification.
5. Mechanical and Electrical
Underfloor heating, new radiators, electrical distribution, lighting design, and any smart home integration. South London planning authorities increasingly expect high energy performance, which means good insulation and efficient heating systems are no longer optional extras.
MEP works: £10,000 to £20,000.
6. Professional Fees
Architect, structural engineer, planning application, building control, and party wall surveyor if required. These are separate from construction costs and typically add 12 to 18 per cent to the build cost.
Professional fees: £10,000 to £20,000 on a project in this range.
- The Full Cost Breakdown at a Glance
Cost Category Typical Range
Groundworks and structure £18,000 £30,000
Roof and glazing £8,000 £20,000
External walls and openings £12,000 £22,000
Internal fit-out £20,000 £40,000
Mechanical and electrical £10,000 £20,000
Professional fees (approx.) £10,000 £20,000
TOTAL ESTIMATE £78,000 £152,000
Note: VAT at 20% applies to construction works and professional fees. This is additional to the figures above.
What Is Not Included in Most Quotes and Should Be
The gap between an initial quote and the final invoice on an extension project is often explained by items that were omitted, assumed, or simply not discussed at the outset. Here is what you should always ask about explicitly:
VAT always confirm whether figures are quoted inclusive or exclusive
Party wall surveyor fees mandatory if your extension is within three metres of a neighbour’s foundation
Kitchen supply contractors price installation; kitchen units are often a separate client supply
Appliances virtually never included in a build contract
Contingency we recommend clients hold 10 to 15 per cent of the build cost in reserve
Making good existing decorations repainting adjoining rooms, repairing disturbed plaster
External landscaping reinstatement of garden areas disturbed during construction
A detailed, itemised contract schedule is the single best protection a client has against cost overruns. If a contractor quotes you a single lump sum with no breakdown, ask for the detail before you sign anything.
How Long Does a Rear Extension Take?
From the point of signing a building contract, a rear single-storey extension typically takes 10 to 20 weeks on site. Add to that the pre-construction phase planning permission (8 to 13 weeks if required), building regulations, structural drawings, and contractor procurement — and you should plan for a total project timeline of 9 to 14 months from initial brief to completion.
Projects that run over time are almost always explained by one of three things: inadequate pre-construction planning, unforeseen ground conditions, or scope changes during the build. The first of these is entirely preventable.
Do I Need Planning Permission?
Many rear single-storey extensions fall within Permitted Development rights and do not require a full planning application. However, Permitted Development rules have specific limits on depth, height, and materials, and they do not apply to all properties. Listed buildings, properties in conservation areas, and some leasehold properties have additional restrictions.
Our strong advice: confirm your planning position with your architect before spending money on detailed drawings. A pre-application enquiry to your local authority costs very little and removes uncertainty.
The following areas have a high concentration of our project work:
London Borough of Croydon
London Borough of Lambeth
London Borough of Southwark
Royal Borough of Kingston upon Thames
London Borough of Merton
Surrey – Reigate, Sutton, and Epsom areas
Why Does It Cost More in South London Than Online Guides Suggest?
Most national pricing guides are built on data from lower-cost regions. South London has higher labour costs, more complex planning environments, higher material delivery costs, and a greater prevalence of older housing stock that presents structural complications.
Any contractor pricing a South London extension against a national average is either working with very thin margins which creates risk for you — or the comparison is not like-for-like.
We price our work to reflect the actual cost of delivering it properly, in this region, with the quality of subcontractors and materials we specify. We would rather be honest about price upfront than have a difficult conversation six weeks into a project.
How to Choose the Right Contractor
Price should not be the deciding factor. Competence, communication, and track record matter more. When comparing contractors, ask the following:
Can you show me three completed extensions of similar scale and specification?
Are you a registered main contractor with your own directly employed workforce?
How do you handle variations and changes to scope during the build?
What contract do you use, and can I see a copy before we agree anything?
Who will be my day-to-day site contact?
Are you insured for public liability, employers’ liability, and contract works?
A contractor who hesitates on any of these questions is telling you something important.
What Happens When You Contact Excel Construction
Getting a quote from Excel Construction is straightforward and deliberately so.
We start by asking you to share your architect’s drawings or a brief description of the project. If you do not have drawings yet, that is fine. We are used to working with clients at an early stage and can help you think through scope before any design fees are committed.
We then visit the site. We do not price extensions from photographs.
From there, our in-house QS produces a fully itemised quotation every line of work broken down by trade and cost category. There are no lump sums, no vague allowances, and no surprises waiting for you six weeks into the build. You see exactly what you are paying for before you sign anything.
What makes us different
Fixed-price contracts. We do not undercut to win work and then recover margin through variations once you are committed. Our price is our price. If something changes, we tell you immediately and agree it in writing before the work is done not after.
Full transparency through Buildertrend. Every client has their own project portal, updated daily with progress photographs, cost tracking, and programme updates. You can check the status of your project at any time, from anywhere. No chasing. No waiting for a call back.
A team that stays. Our site staff have been with us for ten years or more. You are not getting a rotating cast of subcontractors who have never worked together. You are getting people who know how we work, who take pride in what they build, and who will still be here if you need us after completion.
Clients for life. A significant part of our work comes from clients who have used us before or from their children, who grew up watching us work on their family home. That does not happen by accident. It happens because we do what we say we will do, at the price we agreed, to a standard we are proud of.
The things we are proud of
We have delivered projects featured in the Architects Journal, Homes & Gardens, Archello and Dezeen. We work across South London and Surrey on some of the most demanding residential projects in the region. And we have been doing it for over 20 years.
But what we are most proud of is simpler than that: clients who trust us enough to come back.
As Seen In
Our work has been featured in national publications including Homes & Gardens and the Architects Journal.
The Edna Street project in Battersea a complete Victorian villa transformation including basement dig, Crittall glazing, and full interior fit-out was published in Homes & Gardens as an example of how period architecture can be reimagined for contemporary living.
The 158 Purley Downs Road development in Croydon seven new family homes replacing a single dwelling was featured in the Architects Journal as a case study in suburban densification and design quality.
These are not advertorial placements. They are editorial features selected by the publications on merit.
Get a Detailed Quote for Your Rear Extension
Excel Construction delivers rear extensions across South London, Croydon, Surrey, and surrounding areas. We are a fully insured main contractor with over 20 years of experience in high-specification residential construction.
To request a site visit and detailed quotation:
Excel Construction Ltd
148 Addiscombe Road, Croydon
www.excelconstructionuk.co.uk
Prices reflect South London market conditions as at 2026 and are indicative only. All projects are individually assessed.