House Extension Cost Calculator UK

House Extension Cost Guide UK

What's the key thing to understand about Extension Cost Benchmarks 2026?

Typical build costs per square metre of new floor area in the UK (2024): budget single-storey: £1,500-2,000/m². Mid-range single-storey: £2,000-2,800/m². Premium single-storey (bifolds, underfloor heating, high-spec finish): £2,800-4,000/m². Double-storey extensions cost approximately 30-40% less per m² than single-storey (foundations, roof, and drainage are shared across more floor area). London and South East commands a 25-35% premium over Midlands/North prices. Loft conversions: £35,000-60,000 typically, often cheaper per usable square metre than a ground-floor extension since the roof structure is already in place.

What should I know about Professional Fees to Budget?

Professional fees are often overlooked in extension budgets. Architect design and planning drawings: 5-10% of build cost (typically £3,000-8,000). Structural engineer calculations: £1,500-3,500. Party Wall Agreement (if extending within 3m of a neighbour's foundation): £800-2,500 (surveyor fee, both surveyors if neighbour appoints their own). Planning application: £258 for a householder application (if not covered by permitted development). Building Regulations application: £400-900 for a full plans submission, which is separate from and in addition to any planning permission fee.

What's the difference between Permitted Development and Planning Permission?

Many single-storey extensions qualify as Permitted Development (PD) and do not require a planning application. PD limits for houses (England): rear extension up to 4m from rear wall (detached) or 3m (attached). Maximum height 4m. No more than half the garden area. Side extensions in PD are more restricted. PD does not apply in Conservation Areas, Areas of Outstanding Natural Beauty, or if previous PD rights have been removed by condition. Always check with your local planning authority before assuming permitted development rights apply, since restrictions vary by property and location and a mistaken assumption can mean costly retrospective applications.

What's the difference between Value Added and Cost?

Extensions typically add more value than they cost in many areas. A well-designed kitchen extension adds approximately 5-10% to house value. An extra bedroom adds 8-12% in most markets. However: the return depends heavily on whether the extended house remains proportionate to the street — extending a 2-bed terrace to 4 beds in a 2-bed terrace street creates an over-improved property. The best return comes from extensions that address clear functional gaps: adding a third bedroom to a two-bed house, or a second bathroom to a home that only has one, tend to add the most value relative to their cost.

What do I need to know about Party Wall Agreements?

If an extension involves building on or near a boundary shared with a neighbouring property, or cutting into a shared party wall, the Party Wall etc. Act 1996 may require formal notice to be served on the neighbour before work starts, separate from and in addition to any planning permission or building regulations approval. This applies to common extension scenarios including building a new wall up to or astride the boundary line, excavating foundations within a set distance of a neighbour's building (commonly within 3 metres if the new foundations go deeper than the neighbour's, or within 6 metres depending on the angle and depth involved), and cutting into an existing shared wall to tie in a new structure. The neighbour has a statutory right to appoint their own surveyor if they don't agree to the works as proposed, and the cost of resolving a party wall matter, including any surveyor's fees, can fall partly or wholly on the person doing the building work depending on the circumstances. Because this process has statutory notice periods, it should be factored into the project timeline early, since starting notifiable work without proper notice can lead to delays, disputes, or in rare cases a court injunction to halt work.

What should I know about VAT on Extensions?

Most residential extension projects in the UK are subject to the standard rate of VAT on both materials and labour, which should be factored into the cost estimate from this calculator since quotes from builders sometimes quote figures excluding VAT, making them look lower than the real total cost. There are specific, narrower exceptions where a reduced or zero VAT rate can apply: converting a non-residential building into a home, or substantially reconstructing a derelict dwelling that's been empty for a set minimum period, can in some circumstances qualify for the reduced 5% rate rather than the standard rate, and new-build self-contained annexes built for a dependent relative can occasionally qualify for VAT relief under specific conditions, though a typical rear or side extension to an existing, occupied home does not usually qualify for any reduced rate. Given the complexity and strict conditions around these exceptions, anyone expecting to qualify for reduced VAT on an extension project should confirm their specific circumstances with an accountant or HMRC guidance before relying on it in a budget, rather than assuming a reduced rate applies by default.

How Do Single-Storey and Double-Storey Extension Costs Compare?

A double-storey extension costs more in total than a single-storey extension of the same footprint, but considerably less per square metre of additional floor area gained, because much of the most expensive work — foundations, groundwork, and the roof — is largely shared between the ground and first floor rather than duplicated. Foundations for a double-storey extension do need to be somewhat more substantial than for a single-storey extension of the same footprint, to support the additional structural load, but this increase is relatively modest compared with the floor area gained by adding a second storey. The roof, often one of the costlier elements of a single-storey extension relative to its size, is effectively "shared" across both floors in a double-storey version, meaning the cost of roofing per square metre of total new floor area drops significantly when a second storey is added rather than building two separate single-storey extensions. This is why many homeowners planning a single-storey extension, especially a larger one, find that adding a second storey — even one not immediately needed — can represent much better long-term value, since the structural groundwork that's the most disruptive and foundation-dependent part of the project is already being paid for regardless of how many storeys sit on top of it. The trade-off is a higher total upfront cost and, depending on design, a greater likelihood of needing full planning permission rather than falling under permitted development rules that often cover simpler single-storey rear extensions.

How Do Costs Compare by Extension Type? (Cost per Square Metre)

Build cost per square metre varies significantly by the shape and type of extension, not just its size. The table below gives a general sense of how typical UK build costs compare across common extension formats — these are indicative ranges, not fixed quotes, and actual costs depend heavily on specification, location, ground conditions and the contractor used.

Worked example: a homeowner is pricing a 24m² single-storey rear extension (6m x 4m) at a mid-range specification of around £2,400/m². Build cost alone comes to roughly 24 x £2,400 = £57,600. On top of that, professional fees (architect, structural engineer, Building Regulations application) at a typical 6-10% of build cost add approximately £3,500-£5,800, and a sensible contingency allowance of 10-15% for unforeseen issues (drainage diversions, hidden damp, ground conditions) adds a further £5,800-£8,600. All-in, a realistic total budget for this extension typically lands somewhere around £67,000-£72,000 rather than the headline build-cost figure alone — which is why the professional fees and contingency should always be added on top of a per-m² build estimate, not treated as already included in it.

What Else Can Push an Extension's Price Above the Per-m² Estimate

A per-square-metre figure assumes a straightforward build on a reasonably accessible, level site with standard ground conditions — several common factors can push the real cost noticeably higher. Demolition and clearance of an existing structure (a lean-to, old conservatory, or garage being replaced) adds cost before the new build even starts, and the price varies a lot depending on whether the material can be reused or has to be skipped and disposed of. Diverting existing drains, gas, water or electricity runs that happen to sit under the new footprint is a frequently underestimated cost, since it's often only discovered once groundwork starts and the exact position of underground services becomes clear. Poor site access — a narrow side passage, no space for a skip or mixer lorry, or a long carry for materials — increases labour time and sometimes requires smaller, more expensive deliveries or hand-digging of foundations rather than machine excavation. Sloping or poor ground conditions (clay that needs deeper foundations, or a site requiring piled foundations rather than standard strip footings) can add a meaningful premium to the groundwork stage specifically, independent of the extension's size or specification above ground. Large open-plan designs that remove a significant length of load-bearing wall typically need a wider structural steel beam, and steel beam cost rises more than linearly with the span it needs to cover, so a wide, single-span open-plan rear extension can carry a meaningfully higher structural cost than a narrower design of the same floor area. None of these are reflected in a simple cost-per-m² figure, which is why a fixed-price quote from a surveyor who has actually seen the site is always more reliable than scaling a per-m² average by floor area alone.