How Much Does a House Extension Cost in Hertfordshire?

Most online cost guides quote national averages. Those figures are useful as a starting point, but they do not reflect the reality of building in Hertfordshire and North London, where labour costs, material supply chains, and planning authority requirements all differ from the national norm.

Hertfordshire sits in a high-cost zone for construction. Labour rates are broadly comparable to inner London for specialist trades — structural engineers, steel fabricators, and experienced groundworkers — while material costs reflect proximity to London's supply network. A single-storey rear extension that costs £28,000–£35,000 in the Midlands will typically cost £40,000–£60,000 in Borehamwood, Radlett, or St Albans.

The figures in this guide are based on projects completed by TCM Building & Maintenance across Hertfordshire and North London in 2024 and 2025. They reflect actual tender prices, not theoretical estimates derived from cost databases.

A single-storey rear extension is the most common project type in Hertfordshire. The typical cost range for a standard rear extension — 4–6 metres deep, full-width or near-full-width — is £40,000–£65,000 for a standard specification and £65,000–£95,000 for a high specification with bifold doors, underfloor heating, and a high-end kitchen.

The key cost variables at this scale are: the depth of the extension (every additional metre adds approximately £3,500–£5,000 to the structural and roofing cost); the roof type (a flat roof with a lantern light costs less than a pitched roof but requires more maintenance over time); the glazing specification (bifold doors range from £4,500 for aluminium to £12,000+ for thermally broken aluminium with triple glazing); and the internal fit-out (a basic kitchen installation adds £8,000–£15,000, while a bespoke fitted kitchen adds £20,000–£50,000).

Building regulations compliance — structural calculations, building control inspections, and a completion certificate — adds approximately £1,500–£2,500 to the project cost. This is non-negotiable and must be included in any budget.

A double-storey rear extension adds two floors simultaneously, making it the most cost-effective way to significantly increase a property's floor area. The typical cost range is £75,000–£130,000 for a standard specification and £130,000–£180,000 for a high specification.

The cost-per-square-metre for a double-storey extension is lower than for a single-storey, because the foundations, scaffolding, and roof are shared across two floors rather than one. A double-storey extension adding 30m² on each floor (60m² total) will typically cost less per square metre than a single-storey extension adding 30m².

Planning permission is required for double-storey extensions — they fall outside permitted development rights. In Hertfordshire, a planning application costs £258 (2025 fee) and typically takes eight to ten weeks to determine. TCM manages the planning application process as part of the project, including the preparation of drawings and the planning statement.

Side return extensions fill the narrow gap between a semi-detached or terraced house and the boundary fence, typically adding 2–3 metres of width to the ground floor. They are particularly common in Victorian and Edwardian terraces across Barnet, Hendon, and Finchley. The typical cost is £35,000–£55,000 for a side return alone, or £60,000–£90,000 when combined with a rear extension to create a full wrap-around.

Side return extensions often require a party wall agreement with the neighbouring property, because the works are close to or on the shared boundary. The party wall process adds £800–£2,000 to the project cost (for a party wall surveyor's fee) and typically takes four to eight weeks. TCM advises on the party wall process at the outset of every project and can recommend experienced party wall surveyors.

Wrap-around extensions — combining a rear extension with a side return — are the most transformative ground-floor project available to semi-detached and terraced houses. They typically add 25–40m² of floor area and allow a complete reconfiguration of the ground floor into an open-plan kitchen-diner-living space. The cost range is £65,000–£110,000 for a standard specification.

A loft conversion adds a bedroom and bathroom without touching the garden, making it the preferred option for properties with limited outdoor space or planning constraints. Costs vary significantly by conversion type:

Velux (rooflight) conversion: £25,000–£40,000. No structural changes to the roofline — Velux windows are installed in the existing roof slope. Suitable for properties with adequate head height (minimum 2.2 metres at the ridge).

Dormer conversion: £40,000–£65,000. A box-shaped extension to the rear roof slope creates a full-height room with vertical walls. The most common conversion type in Hertfordshire. Typically falls within permitted development rights on semi-detached and detached properties.

Hip-to-gable conversion: £50,000–£75,000. Converts the sloping hip end of the roof into a vertical gable, adding significant floor area. Required on detached and some semi-detached properties where a dormer alone would not provide enough space.

Mansard conversion: £65,000–£95,000. The most extensive conversion type — the entire rear roof slope is replaced with a near-vertical structure. Maximises floor area but requires planning permission. Common in conservation areas where a dormer would be refused.

Several factors push costs above the standard range, and understanding them in advance allows you to budget accurately and make informed decisions about specification.

Ground conditions. Clay soils — prevalent across much of Hertfordshire — require deeper foundations than sandy or gravelly ground. In areas with mature trees, tree root protection zones may require specialist foundation design (pile and beam, or raft foundations), adding £5,000–£15,000 to the groundworks cost.

Structural steelwork. Open-plan layouts require steel beams to carry the load of the structure above. A single steel beam installation typically costs £2,500–£5,000 including structural engineer's calculations, beam supply, and installation.

Conservation area restrictions. Properties in conservation areas — including parts of Radlett, Barnet, and Elstree — must use materials that match the existing building. Handmade clay roof tiles, lime render, and timber windows all cost more than standard alternatives.

Party wall works. Where the extension is close to or on the shared boundary with a neighbour, a party wall agreement is required. If the neighbour appoints their own surveyor, you are responsible for both surveyors' fees — typically £1,500–£3,500 in total.

A complete extension budget includes more than the builder's contract price. The following items are frequently overlooked and should be included from the outset.

Architectural drawings: £1,500–£3,500 for planning drawings; £2,500–£5,000 for full working drawings. Required for planning applications and building regulations submissions.

Structural engineer: £800–£2,000 for calculations and drawings. Required for building regulations approval on any extension with structural steelwork or significant loadings.

Planning application fee: £258 (2025 rate) for a householder application.

Building regulations: £800–£1,500 for a full plans application. This covers the building control officer's inspections throughout the build and the issue of a completion certificate at the end.

Party wall surveyor: £800–£1,500 for a single surveyor acting for both parties; £1,500–£3,500 if the neighbour appoints their own surveyor.

Contingency: A 10–15% contingency on the build cost is standard practice. Unforeseen ground conditions, structural discoveries during demolition, and material price movements are the most common sources of cost overrun.

Most building work on existing residential properties is subject to VAT at the standard rate of 20%. This applies to labour and materials supplied by the contractor. VAT is not included in the cost ranges quoted above — add 20% to arrive at the VAT-inclusive figure.

There are limited exceptions. Converting a non-residential building to residential use is zero-rated for VAT. Certain energy-saving materials — insulation, solar panels, heat pumps — are subject to a reduced rate of 5% when supplied and installed as part of a residential project. Your contractor should be VAT-registered and should provide a VAT invoice showing the VAT charged separately.

Online cost calculators and per-square-metre rates give a rough order of magnitude, but they cannot account for the specific conditions of your property. The only way to get an accurate figure is to have a builder carry out a site survey and produce a detailed specification.

A detailed specification breaks the project down into individual work packages — groundworks, structural frame, roofing, external envelope, internal fit-out, M&E — and prices each one separately. This approach makes it possible to compare quotes from different contractors on a like-for-like basis, and to identify where one contractor is pricing significantly above or below the others.

TCM offers free site surveys across Hertfordshire and North London. The survey includes an assessment of the ground conditions, the structural requirements, and the planning position, and produces a detailed cost estimate within five working days.

Most homeowners finance extensions through one of three routes: savings, remortgaging, or a further advance from their existing lender. Each has different cost implications.

Remortgaging to release equity is the most common route for projects over £50,000. If your property has increased in value since you took out your mortgage, you may be able to borrow against that equity at your mortgage interest rate — typically lower than a personal loan or credit card. The arrangement fee and legal costs of remortgaging add £1,000–£2,500 to the total cost.

A further advance from your existing lender avoids the need to remortgage but may be at a higher interest rate than your existing mortgage. For smaller projects — under £30,000 — a personal loan may be more cost-effective than remortgaging, particularly if the arrangement fees and legal costs would offset the interest rate saving.

TCM Building & Maintenance provides fixed-price contracts for all extension projects. The contract price is agreed before work starts and does not change unless the scope of work changes — which is documented in a formal variation order signed by both parties.

Our pricing process starts with a free site survey. We assess the ground conditions, the structural requirements, and the planning position, and produce a detailed specification. That specification is the basis for the contract price. We do not provide ballpark figures over the phone — the site survey is the only way to price accurately.

We are transparent about what is and is not included in the contract price. Architectural drawings, structural engineer's fees, planning application fees, and building regulations fees are typically excluded from the build contract and quoted separately. We will advise on the total project cost — including all professional fees — at the outset, so there are no surprises.

How much does a single-storey extension cost per square metre in Hertfordshire?

A single-storey rear extension in Hertfordshire typically costs £1,800–£2,500 per square metre for a standard specification, or £2,500–£3,500 per square metre for a high specification with bifold doors, underfloor heating, and a premium kitchen. These figures include all building work but exclude VAT, architectural fees, structural engineer's fees, and planning fees.

Does a house extension add value in Hertfordshire?

A well-executed extension in Hertfordshire typically adds 10–20% to the property's market value. Adding a fourth bedroom to a three-bedroom semi-detached in Borehamwood or Radlett consistently produces strong value uplift because demand for four-bedroom family homes in these areas is high.

Do I need planning permission for a house extension in Hertfordshire?

Many single-storey rear extensions in Hertfordshire can be built under permitted development rights without planning permission. The standard permitted development limit is 4 metres deep for detached houses and 3 metres for semi-detached and terraced houses. Under the neighbour consultation scheme, these limits extend to 8 metres and 6 metres respectively. Double-storey extensions, side extensions in some areas, and any extension in a conservation area or on a listed building require planning permission.

How long does a house extension take to build?

A single-storey rear extension typically takes 10–16 weeks from start to completion. A double-storey extension takes 16–24 weeks. These timescales assume planning permission has been obtained before work starts. The planning process for a householder application in Hertfordshire typically takes 8–10 weeks.

What is a contingency and how much should I allow?

A contingency is a budget reserve for unforeseen costs. On an extension project, the most common sources of unforeseen cost are: unexpected ground conditions (requiring deeper or specialist foundations), structural discoveries during demolition, and material price movements during a long build programme. A 10–15% contingency on the build cost is standard practice in Hertfordshire.