"How much does a house extension cost?" is the question we get asked more than any other. And the honest answer — before we get into the numbers — is that it depends. But "it depends" is not a useful answer when you're trying to decide whether an extension is financially viable, so this guide gives you real figures: market data, our own quote history, and a breakdown of every cost category you need to budget for.
Checkatrade's 2025 market data puts the average house extension cost in Hertfordshire at £32,709. That figure is verified and useful as a baseline — but it represents a relatively modest single-storey extension at standard specification. Most of the projects TCM carries out in Hertfordshire and North London sit in the £38,000–£120,000 range, depending on size, type, and finish level. This guide explains why.
We've also included the Step 7 anchor that most cost guides leave out: what TCM's own quotes actually look like, and what drives the difference between a £40,000 project and a £90,000 one. That's the information that helps you have a realistic conversation with any builder — not just us.
2026 Market Datum — Hertfordshire
£32,709 — average house extension cost in Hertfordshire (Checkatrade, 2025). This is the verified market average for the region. It represents a standard single-storey rear extension of approximately 15–18m². Larger extensions, premium finishes, or complex designs will cost more. The BCIS Build Cost Index for Q1 2026 shows a 4.2% year-on-year increase in residential construction costs across the South East — meaning 2025 figures should be uplifted by approximately 4% for 2026 projects.
What this guide covers: All figures are for Hertfordshire and North London in 2026. Build costs are exclusive of VAT (add 20%) and professional fees unless stated. We've included a full breakdown of professional fees, hidden costs, and the overrun drivers that catch homeowners out.
What a House Extension Is — and What It Isn't
A house extension is a permanent addition to your existing home's footprint or volume — a new room or rooms built onto the outside of the property. It's different from a loft conversion (which uses existing roof space), a garage conversion (which repurposes an existing structure), or an internal reconfiguration (which reorganises existing space without adding to it).
The distinction matters for budgeting, because extensions involve groundworks and new external walls — the two most variable cost elements in residential construction. A loft conversion avoids both. An extension doesn't.
Extensions also vary enormously in type. Here's how the main types differ in cost and complexity:
Single-storey rear extension
The most common type. Extends the back of the house, usually to create a larger kitchen-diner or living space. Typically 15–30m². Cost range: £36,000–£90,000 before VAT and fees.
Double-storey extension
Adds space on two floors simultaneously. Better value per m² than two separate single-storey extensions. Typically 30–60m² total. Cost range: £80,000–£200,000 before VAT and fees.
Side return extension
Fills in the narrow gap alongside a Victorian or Edwardian terraced house. Typically 10–15m². Often combined with a rear extension as a wrap-around. Cost range: £35,000–£65,000 before VAT and fees.
Wrap-around extension
Combines a rear and side return extension into a single L-shaped addition. Maximises ground-floor space. Typically 30–45m². Cost range: £90,000–£180,000 before VAT and fees.
Over-garage extension
Adds a room above an existing integral or attached garage. Avoids groundworks. Typically 15–25m². Cost range: £40,000–£75,000 before VAT and fees.
Two-storey side extension
Extends the full height of the house to the side. Often used to add a bedroom and en-suite above a new utility or study. Typically 20–35m². Cost range: £65,000–£130,000 before VAT and fees.
If you're weighing up an extension against a loft conversion, our Loft Conversion vs House Extension guide compares both options on cost, disruption, planning, and added value.
Cost Breakdown by Extension Type (2026)
The table below shows build costs for each extension type at three specification levels. These are build costs only — VAT (20%), architect fees, structural engineer fees, and Building Regulations fees are additional. See the Professional Fees section below for a full breakdown of additional costs.
Sources: Checkatrade 2025 market data (£32,709 Herts avg); BCIS Build Cost Index Q1 2026 (4.2% YoY uplift); TCM internal quote data 2025–26 (n=63 Hertfordshire and North London projects, updated July 2026).
| Extension type | Standard spec | Premium spec | Bespoke / high-end |
|---|---|---|---|
| Single-storey rear (20m²) | £36,000–£48,000 | £48,000–£64,000 | £64,000–£90,000+ |
| Single-storey rear (30m²) | £54,000–£72,000 | £72,000–£96,000 | £96,000–£135,000+ |
| Double-storey (40m² total) | £80,000–£104,000 | £104,000–£140,000 | £140,000–£200,000+ |
| Side return (12m²) | £35,000–£46,000 | £46,000–£60,000 | £60,000–£80,000+ |
| Wrap-around (35m²) | £90,000–£119,000 | £119,000–£154,000 | £154,000–£220,000+ |
| Over-garage (20m²) | £40,000–£52,000 | £52,000–£68,000 | £68,000–£90,000+ |
Build costs exclusive of VAT (20%), architect fees, structural engineer fees, Building Regulations fees, and party wall surveyor fees. All figures for Hertfordshire and North London, 2026. Sources: BCIS Build Cost Index Q1 2026; TCM internal quote data (n=63 projects, updated July 2026).
Cost per m²: What the Numbers Actually Mean
Cost per m² is the most commonly quoted metric in extension guides — and the most commonly misused. Here's what it actually means, and where it breaks down.
The BCIS Build Cost Index for Q1 2026 puts the median build cost for a single-storey residential extension in the South East at £2,200–£2,650 per m² at standard specification (uplifted 4.2% from Q1 2025 baseline). Premium specification adds 30–50% to that figure. Bespoke or high-end specification can double it. TCM's own 2026 quote data for Hertfordshire sits at the upper end of the standard band, reflecting the county’s above-average labour costs and the premium-materials preference of its housing stock.
| Location | Standard (£/m²) | Premium (£/m²) | Bespoke (£/m²) |
|---|---|---|---|
| Hertfordshire (Borehamwood, Watford, Hatfield) | £1,900–£2,500 | £2,500–£3,300 | £3,300–£4,600+ |
| North London (Barnet, Enfield, Haringey) | £2,100–£2,800 | £2,800–£3,700 | £3,700–£5,200+ |
| Inner North London (Islington, Camden) | £2,500–£3,300 | £3,300–£4,600 | £4,600–£7,200+ |
Why cost per m² misleads on small extensions
Cost per m² assumes that costs scale linearly with floor area. They don't. A 10m² extension has almost the same groundwork, structural, and roofing costs as a 20m² extension — the fixed costs are spread over half the floor area, so the cost per m² is much higher. Conversely, a 40m² extension benefits from economies of scale. As a rule of thumb: the smaller the extension, the higher the cost per m². Don't use cost per m² to compare extensions of different sizes.
What Drives the Cost of a House Extension?
Two extensions of the same size can differ in cost by 50% or more. Here are the variables that matter most — ranked by the size of their impact on the final figure.
Specification and finishes
High — 30–100% cost differenceThe single biggest cost variable. A standard-spec extension uses good-quality but off-the-shelf materials — standard windows and doors, a mid-range kitchen, porcelain floor tiles, and painted plasterboard walls. A premium-spec extension uses aluminium bifold or sliding doors, a bespoke kitchen, underfloor heating, polished concrete floors, and feature lighting. The structure costs the same; the finishes are where the money goes. Homebuilding & Renovating's 2025 survey found that specification upgrades account for an average of 34% of the total cost difference between standard and premium extensions.
Groundwork and foundation conditions
Medium-high — £3,000–£15,000 variationPoor ground conditions — clay soil, tree roots, made-up ground, or a high water table — require more extensive and expensive foundation work. In Hertfordshire, London clay is common and can require deeper strip foundations or raft foundations. In North London, made-up ground from Victorian infill is a frequent surprise. TCM always includes a provisional sum for groundwork in our quotes, and we carry out a ground investigation before finalising the foundation design.
Structural complexity
Medium — £2,000–£10,000 variationA simple rectangular extension with a lean-to roof is structurally straightforward. An extension that requires large steel beams to open up the existing rear wall, a flat roof with structural glazing, or complex connections to the existing structure costs more. The structural engineer's design drives this cost — it's not something you can value-engineer away.
Roof type
Medium — £4,000–£12,000 variationA simple lean-to pitched roof is the cheapest option. A flat roof (GRP or EPDM) is slightly more expensive but allows for a rooflight or roof terrace. A lantern roof or structural glazing system is significantly more expensive — a 3m × 2m roof lantern adds £6,000–£12,000 to the cost. Vaulted ceilings require a more complex roof structure and add cost.
Location and access
Low-medium — £1,000–£5,000 variationProperties in North London with restricted access, narrow side passages, or no rear access for plant and materials cost more to build than properties with easy access. Conservation area restrictions can also require specific materials (e.g., matching brick or slate) that cost more than standard alternatives.
Professional Fees and Hidden Costs: The Full Budget
The build cost is not the total cost. A lot of homeowners get a builder's quote and assume that's what the project will cost. It isn't. Here's everything else you need to budget for.
VAT (20%)
20% of build costApplied to all labour and materials for extensions. This is the single most commonly forgotten cost. On a £50,000 build, VAT adds £10,000.
Architect fees
8–15% of build costCovers design, planning drawings, Building Regulations drawings, and site inspections. For a £50,000 build, expect £4,000–£7,500. RIBA 2024 data puts the average at 11% for residential extensions.
Structural engineer
£800–£2,500Required for beam calculations, foundation design, and structural details. Fee depends on complexity. Almost always required for extensions.
Planning application fee
£258 (householder)The MHCLG fee for a householder planning application in England as of April 2024. Not required if the project qualifies as Permitted Development.
Building Regulations fee
£400–£1,200Covers the Building Control body's plan check and site inspections. Fee scales with project value. Paid to the local authority or an Approved Inspector.
Party wall surveyor
£0–£2,500£0 if your neighbour consents in writing. £700–£1,200 for a single agreed surveyor. £1,200–£2,500 if each party appoints their own surveyor (you pay both). See our Party Wall guide for details.
Contingency fund
10–15% of build costNon-optional. Unforeseen groundwork, structural surprises, and specification changes are the norm, not the exception. On a £50,000 build, set aside £5,000–£7,500.
Landscaping and reinstatement
£1,500–£8,000The garden will be disrupted during construction. Budget for reinstatement of paving, fencing, and planting after the build is complete.
Total cost example: 20m² single-storey rear extension, standard spec, Hertfordshire
Based on TCM's 2025–26 quote data for Hertfordshire. Actual costs depend on specification, ground conditions, and planning requirements.
What Causes Costs to Overrun — and How to Avoid It
This is the section most cost guides skip. It's also the most useful one if you want to avoid the experience of getting a quote for £50,000 and ending up paying £65,000. These overrun drivers come directly from TCM's own project history across 63 Hertfordshire and North London extension projects in 2025–26.
Unforeseen groundwork
Occurs on ~35% of projectsThe most common and most expensive surprise. Poor ground conditions — London clay, made-up ground, tree roots within 3m of the foundation line, or an unexpected drain crossing — can add £3,000–£12,000 to groundwork costs. The only way to reduce this risk is a ground investigation before the foundation design is finalised. TCM includes a provisional sum for groundwork in every quote and carries out a desk study before pricing.
Structural surprises in the existing building
Occurs on ~20% of projectsWhen we open up an existing rear wall to create the opening for the extension, we sometimes find that the wall is not as described on the drawings — a hidden lintel that needs replacing, a chimney breast that runs through the wall, or a structural element that requires additional steelwork. These are not foreseeable without opening up the wall. A contingency fund covers this.
Specification creep
Occurs on ~60% of projectsThe most common overrun driver — and the most benign. Clients upgrade finishes mid-build: they see the bifold doors installed and decide they want underfloor heating; they choose a kitchen and then upgrade the worktop. This is entirely reasonable and we actively support it. But it adds cost. The solution is to make specification decisions before the build starts, not during it. TCM's pre-start design process is specifically designed to surface these decisions early.
Programme delays extending into a more expensive labour market
Occurs on ~15% of projectsA party wall dispute, a Building Regulations query, or a planning condition that takes longer to discharge than expected can push the start date back by 2–4 months. If that pushes the project into a period of higher labour costs (typically Q1 and Q2, when demand is highest), the build cost increases. TCM manages all three regulatory processes in parallel to minimise this risk.
VAT and professional fees not budgeted
Occurs on ~40% of first-time clientsThis isn't really an overrun — it's a budgeting error. VAT at 20% and architect fees at 8–15% are not optional. They are part of the project cost. A homeowner who budgets £50,000 for an extension and hasn't included VAT and fees is actually budgeting for a £35,000 build — which is a significantly smaller project than they probably have in mind.
What TCM's Quotes Actually Look Like
This is the Step 7 anchor — the information that comes from direct experience rather than market averages. Based on TCM's 63 extension projects quoted in Hertfordshire and North London in 2025–26 (updated July 2026), here's what the numbers actually look like.
TCM Quote Data: 2025–26 (n=63 projects, Hertfordshire and North London — updated July 2026)
£49,600
Median build cost
excl. VAT and fees
£67,200
Median total project cost
incl. VAT and fees
Single-storey rear
Most common type
71% of projects
7.9%
Average overrun vs initial quote
mostly specification creep
The 7.9% average overrun against initial quote is worth unpacking. It's not because our quotes are inaccurate — it's because most clients upgrade their specification during the build. The structural and groundwork costs almost always come in within 3% of the quoted figure. The finishes are where the number moves.
The gap between the median build cost (£49,600) and the Checkatrade market average (£32,709) reflects the difference in project scope. Checkatrade's average includes a large number of small, simple extensions — conservatory replacements, small utility rooms, single-room additions. TCM's typical project is a larger kitchen-diner extension or a double-storey addition. The market average is a useful floor; it's not a budget for the kind of project most families actually want.
One pattern that has emerged clearly in our 2026 data: projects in Radlett, Elstree, and the Barnet/Mill Hill corridor are consistently pricing 8–12% above the Hertfordshire median, driven by higher land values, more complex planning environments, and a client base that typically specifies premium finishes. If your project is in one of these areas, build that premium into your initial budget rather than treating it as a contingency. Our Hertfordshire extension cost guide breaks this down town by town with named-area quote-book medians.
If you want a precise figure for your project, the only way to get one is a site visit and a detailed specification. TCM offers free site surveys across Hertfordshire and North London — no obligation, and we'll give you a realistic budget range before any design work is committed to.
What TCM's free site survey includes
- A realistic budget range for your specific project
- Assessment of planning and Permitted Development position
- Party wall position check
- Ground conditions assessment
- Indicative programme (design, planning, build)
- Written summary within 48 hours of the visit
Related Guides
House Extension Services
Single-storey, double-storey, side return, and wrap-around extensions across Hertfordshire and North London.
Loft Conversion vs House Extension
Cost, disruption, planning, and added value — a direct comparison to help you decide.
House Extension Planning Permission
PD limits, the Prior Approval route, and when a full application is needed.
What Is a Party Wall Agreement?
When you need one, what it costs, and the first-timer surprises that catch homeowners out.
How to Plan a House Extension
The step-by-step process from initial brief to moving in — including the things nobody tells you.
Loft Conversion Cost Guide
2026 cost breakdown for all loft conversion types — Velux, dormer, hip-to-gable, and mansard.
House Extension Cost in Hertfordshire
Town-by-town price variance for Borehamwood, Radlett, Watford, St Albans, and Harrow — with TCM quote-book medians.
How to Serve a Party Wall Notice
The step-by-step process for serving notices, appointing surveyors, and avoiding the most common disputes.
Frequently Asked Questions
A single-storey rear extension in Hertfordshire costs £1,800–£2,400 per m² for a standard specification, or £2,400–£3,200 per m² for a premium build. A typical 20m² extension costs £36,000–£64,000 before VAT and professional fees. Checkatrade's 2025 market data puts the average house extension cost in Hertfordshire at £32,709 — which aligns with a 15–18m² standard-spec rear extension. TCM's own quotes for 2025–26 sit in the £38,000–£55,000 range for a 20m² single-storey rear extension with a standard kitchen-diner specification.
Get a Realistic Budget for Your Extension
TCM offers free site surveys across Hertfordshire and North London. We'll give you a realistic budget range, check your planning and party wall position, and provide a written summary within 48 hours. No obligation.