If you're planning a rear extension in London and you've started searching for costs, you've probably encountered a wide spread of figures — anywhere from £30,000 to £120,000 or more. That range isn't misleading. It reflects how differently rear extensions can be scoped, finished, and procured. This post gives you an honest breakdown of what drives costs, what to budget at each level, and what to look for in a contractor.
Typical Cost Ranges for a Rear Extension in London (2025)
These are all-in build costs, excluding VAT, architect fees, and planning unless noted:
| Extension Type | Typical Budget Range | |---|---| | Single-storey, basic spec, up to 4m | £35,000 – £55,000 | | Single-storey, mid spec, up to 6m | £55,000 – £75,000 | | Single-storey, high spec (bi-folds, underfloor, open plan) | £75,000 – £110,000 | | Double-storey rear extension | £90,000 – £150,000+ |
London pricing sits above national averages by 20–35%, driven by labour costs, access constraints, skip and plant hire, and material delivery in urban areas. These figures assume a standard London semi-detached or terrace with reasonable site access.
What Affects the Price of a Rear Extension
1. Size and Footprint
The floor area is the single biggest cost driver. Extending 3m out across a 4m-wide kitchen opening is a very different project from a full-width 6m extension across a 7m rear elevation. Every additional square metre adds structural, roofing, and finishing cost.
2. Structural Openings
Most rear extensions involve removing part or all of the existing rear wall to create an open-plan layout. This means steel beams, padstone installation, and temporary propping. A simple single RSJ (rolled steel joist) might add £3,000–£6,000. Complex multi-span arrangements across an already open rear can push structural steelwork to £10,000–£15,000.
3. Roof Type
A flat roof with a standard build-up (warm deck or inverted system) is the most common approach for single-storey extensions. Lantern rooflight or glazed ridge additions cost more — a quality rooflight unit alone runs £2,000–£5,000 installed. Pitched roof extensions cost more to build but can look more sympathetic to the original property.
4. Glazing and Doors
Bifold and sliding door systems are the most requested feature in rear extensions. Aluminium systems with thermally broken frames and double glazing start at around £4,000–£6,000 for a 3-panel set and scale significantly for larger spans. Quality here matters — cheap units fail at seals and ironmongery within a few years.
5. Drainage and Groundwork
If the extension footprint crosses existing drainage runs, you'll need a drainage survey and possible diversion. This can add £2,000–£8,000 depending on pipe depth and proximity to the main sewer. It's frequently underestimated in early-stage budgets.
6. Internal Fit-Out
The extension shell alone doesn't give you a functioning kitchen-diner. Budget separately for:
- Kitchen: £8,000–£40,000+ depending on specification
- Underfloor heating: £80–£120/m²
- Flooring, plastering, decoration: £4,000–£10,000 depending on area
Design-and-Build vs. Separate Architect and Builder
Many London homeowners take the traditional route: appoint an architect, get planning, put it out to tender, then manage the builder themselves. This works, but it has a known failure mode — the handover between design and construction is where cost overruns and disputes most often originate. The design doesn't always reflect what's practical to build at the price quoted.
A design-and-build contractor does both under one contract. The advantage is that buildability and cost are considered during design, not after. You deal with one point of contact. The risk is choosing a contractor whose design capability is limited — not all builders who offer "design-and-build" have genuine architectural or structural expertise in-house.
The right question isn't which model to use — it's whether your contractor has the capability to execute it properly.
What to Look for in a Rear Extension Contractor in London
- Principal contractor credentials. Your contractor should be able to take full responsibility for the build, including subcontractor management, Health & Safety, and Building Control liaison.
- Structural coordination. They should work directly with a structural engineer, not just hand you a spec and expect you to commission one separately.
- Fixed-price contracts. Any reputable contractor can provide a detailed, itemised quote. Vague day-rate arrangements on a project of this size are a red flag.
- Planning knowledge. Most single-storey rear extensions in London fall under Permitted Development, but restrictions apply (Article 4 areas, conservation zones, flats, terraces with limited depth). Your contractor should advise on this before you spend money on drawings.
- Accreditations. Look for FMB (Federation of Master Builders), TrustMark, or Checkatrade membership — these require vetting, not just registration.
How RCB Design & Build Approaches Rear Extensions
RCB Design & Build are A2Z Principal Contractors covering Greater London and surrounding counties. Rear extensions are a core part of what we do — from initial feasibility and structural coordination through to full fit-out. We work on fixed-price contracts and manage the entire project, including Building Control, structural engineering coordination, and all trades.
We hold FMB, TrustMark, Checkatrade, and TrustATrader accreditations.
If you're budgeting a rear extension, the most useful first step is a conversation about what you want to achieve. We can give you a realistic cost range before you commit to any design fees.
Contact us:
- Email: contact@rcbgroup.co.uk
- Phone: +44 7359 872594
- Web: [rcbgroup.co.uk](https://rcbgroup.co.uk)