3 Metre House Extension: Costs, Planning and Structural Design (2026)
A 3 metre house extension is one of the most popular home improvements in the UK. It is deep enough to transform a kitchen, dining room or living space, and it almost always falls under permitted development so planning permission is rarely needed. This guide covers realistic 2026 costs by size and finish, what structural work is required, and how the planning and Building Regulations process works.
For a broader look at single storey extension costs across all sizes, see our single storey extension cost guide.
How Much Does a 3 Metre House Extension Cost in 2026?
The cost depends on the width of the extension and the finish. Here are realistic 2026 figures for the most common 3 metre extension sizes.
| Size | Total Cost (2026) | Structural Engineer Fee |
|---|---|---|
| 3m x 3m rear extension | £18,000 to £28,000 | £500 to £700 |
| 3m x 4m rear extension | £24,000 to £36,000 | £550 to £800 |
| 3m x 5m rear extension | £28,000 to £42,000 | £600 to £900 |
| 3m kitchen extension (with fit-out) | £36,000 to £55,000 | £600 to £900 |
| 3m extension with bi-folds | £30,000 to £48,000 | £600 to £900 |
| 3m extension pitched roof | £24,000 to £40,000 | £600 to £900 |
All figures exclude VAT and assume standard UK finishes. London and South East add roughly 20 to 30%. The structural engineer fee covers the full calculation pack including beam sizing, foundation specification and Building Control drawings.
3m x 3m Extension Cost (9m² Total)
A 3m x 3m single storey rear extension is the smallest commonly built extension and one of the most cost-effective ways to add usable space. At 9 square metres it suits a utility room, garden room or small additional dining area.
Typical cost breakdown for a 3m x 3m extension
| Cost Element | Typical Cost (2026) |
|---|---|
| Foundations and groundworks | £3,500 to £6,000 |
| Brickwork and blockwork walls | £3,000 to £5,000 |
| Roof (flat or simple lean-to) | £2,500 to £4,500 |
| RSJ beam for rear wall opening | £900 to £1,400 |
| Windows and external door | £1,200 to £2,500 |
| Plastering, floor, electrics | £3,500 to £6,000 |
| Structural engineer fee | £500 to £700 |
| Building Control fee | £300 to £500 |
Total: £18,000 to £28,000 for a basic to standard finish. A 3m x 3m extension with bi-fold doors or a pitched roof adds £3,000 to £8,000 to this figure.
How Much Does a 3m Kitchen Extension Cost?
A 3 metre kitchen extension is the most popular use of a 3m rear extension and the most structurally involved. The rear wall between the existing kitchen and the new extension almost always carries the floor or roof above it, which means an RSJ steel beam is required whenever that wall is opened up.
Typical 3m kitchen extension costs (3m x 4m or 3m x 5m footprint)
- Extension build cost: £24,000 to £38,000
- Kitchen units and appliances: £6,000 to £15,000
- RSJ beam for the wall opening: £900 to £1,600
- Structural engineer fee: £600 to £900
- Bi-fold or sliding doors (if included): £2,000 to £5,000
- Building Control fee: £350 to £600
Total typical range for a 3m kitchen extension: £36,000 to £55,000 for a 3m x 4m or 3m x 5m footprint with a mid-range kitchen fit-out.
The structural element of a 3m kitchen extension
The opening between the existing kitchen and the 3 metre extension is almost always a load-bearing wall. A structural engineer calculates the correct RSJ steel beam size, specifies the padstones at each bearing end and designs the foundation for the new extension. This calculation pack goes to Building Control before the builder can install the steel or start foundation work. See our guide to structural engineering for extensions for how the approval process works.
Do You Need Planning Permission for a 3 Metre Extension?
Most 3 metre single storey rear extensions fall under permitted development and do not need a planning application. The permitted development rules for rear extensions are:
| Property Type | Max Depth (Permitted Development) | Notes |
|---|---|---|
| Terraced house | 3 metres | 3m is the maximum without prior approval |
| Semi-detached house | 3 metres | 3m is the maximum without prior approval |
| Detached house | 4 metres | A 3m extension is comfortably within limits |
When planning permission IS required for a 3m extension
- The property is in a conservation area, national park or Area of Outstanding Natural Beauty
- The property is listed
- Previous extensions have already used up the permitted development allowance
- The extension would extend beyond the side wall of the house
- The roof of the extension would be higher than the eaves of the main roof
Structural Requirements for a 3 Metre Extension
Every 3 metre extension involves structural work. Here is what a structural engineer designs and why each element matters.
RSJ steel beam for the rear wall opening
If the rear wall of the house is being opened up to create a doorway or wide opening into the extension, that wall is almost certainly load-bearing. The structural engineer calculates the correct RSJ beam size to carry the floor, roof or wall load above the new opening, specifies the padstones at each beam end and checks the masonry below can carry the concentrated load.
For a standard 3 metre wide opening, the RSJ beam costs £900 to £1,400 supplied and installed. A wider opening or one with heavier loads above needs a larger and more expensive beam. See our steel beams for extensions guide.
Foundation design
The new 3 metre extension needs its own foundations to carry its weight to the ground. The depth and type of foundation depends on the soil conditions. In non-clay soils the minimum depth is 450mm. In clay soils Building Regulations require 750mm to 900mm. Near trees, the depth can reach 1.5m to 2.5m or more depending on tree species and distance. A structural engineer specifies the exact foundation depth and width as part of the calculation pack.
Structural engineer fees for a 3 metre extension
Structural engineer fees for a 3 metre extension are typically £500 to £900 for the full calculation pack including beam sizing, foundation specification, padstone design and Building Control drawings. More complex extensions with wider openings, unusual ground conditions or bi-fold door frames sit at the upper end. See our full structural engineer cost guide.
Building Regulations for a 3 Metre Extension
Building Regulations approval is required for every 3 metre extension regardless of whether planning permission was needed. Here is how the process works in practice.
- Appoint a structural engineer. Before anything is submitted to Building Control, you need the structural calculation pack. The structural engineer designs the beam, foundations and any other structural elements, then produces a PDF calculation pack and drawings.
- Submit a Full Plans application to Building Control. You can use your local council or a private approved inspector. Submit the structural engineer's drawings and calculations, your floor plans and your site location plan. Building Control reviews the documents and typically responds within five working days for a straightforward domestic extension.
- Notify Building Control before starting. Once plans are approved, let Building Control know you are about to start. They arrange inspection visits at key stages.
- Building Control inspects foundations before concrete. The inspector visits before you pour the foundation concrete to check depth and width match the approved drawings.
- Building Control inspects steelwork before it is covered up. The inspector checks the RSJ beam, padstones and any connections before the builder closes in the walls and ceiling.
- Final inspection and completion certificate. Once the extension is finished, Building Control issues a completion certificate. Keep this for when you sell the property.
3 Metre Extension Cost by Width and Type
4m x 3m extension cost
A 4m wide by 3m deep extension gives 12 square metres of floor area. This is the most common size for a rear kitchen or dining room extension. Total cost: £24,000 to £36,000 for a standard finish without a kitchen fit-out. With a mid-range kitchen: £36,000 to £50,000.
5m x 3m extension cost
A 5m wide by 3m deep extension gives 15 square metres. This is a good size for an open-plan kitchen-diner or a large living space extension. Total cost: £28,000 to £42,000 for a standard finish. The wider opening between the extension and the house usually requires a longer RSJ beam which adds cost.
6m x 3m extension cost
A 6m wide by 3m deep extension gives 18 square metres of floor area. At this width, the opening between the house and the extension is typically six metres, which requires a heavier steel beam or a goalpost steel frame to carry the load above. Total cost: £35,000 to £55,000. Structural engineer fee: £700 to £1,100.
3m rear extension vs 3m side extension
A 3m side extension extends out from the side of the house rather than the rear. These are slightly more complex structurally because the gable wall at the side is often load-bearing. Side extensions may also require a party wall agreement with a neighbour if they are close to the boundary. Costs are broadly similar to rear extensions but often sit at the upper end of the range due to access difficulties and more complex drainage.
What Affects the Cost of a 3 Metre Extension
- Width of the extension: wider extensions need more materials and a larger, more expensive RSJ beam for the opening.
- Roof type: a flat roof is the cheapest option. A lean-to pitched roof adds £2,000 to £5,000. A full pitched roof adds more.
- Glazing: standard windows are the baseline. Bi-fold or sliding doors add £2,000 to £5,000. A roof lantern adds £1,500 to £4,000.
- Ground conditions: clay soils, soft ground or trees near the extension push foundation depth and cost up significantly.
- Kitchen or bathroom fit-out: these add £6,000 to £20,000 on top of the build cost.
- Location: London and the South East typically add 20 to 30% to labour costs.
- Finish quality: the difference between basic and high-end finishes can be £8,000 to £15,000 on a typical 3 metre extension.
For a full breakdown of cost drivers across all extension sizes, see our single storey extension cost guide and our cost of extensions overview.
FAQ: 3 Metre House Extension
How much does a 3m x 3m extension cost in 2026?
A 3m x 3m single storey rear extension costs £18,000 to £28,000 in total for a standard finish. This includes foundations, brickwork, a simple flat or lean-to roof, the RSJ beam for the rear wall opening, windows, plastering, electrics and the structural engineer fee. A pitched roof or bi-fold doors add £3,000 to £8,000.
Do I need planning permission for a 3 metre extension?
Most 3m rear extensions fall under permitted development and do not need planning permission. The limit is 3m for terraced and semi-detached houses and 4m for detached houses. Exceptions apply in conservation areas and for listed buildings. You always need Building Regulations approval regardless of whether planning permission is needed.
Do I need a structural engineer for a 3 metre extension?
Yes. Building Control requires structural calculations before approving the steelwork in any extension. The structural engineer designs the RSJ beam for the rear wall opening, specifies the foundations and produces the calculation pack. Fees start from £500. See our structural engineer cost guide.
How much does a 3m kitchen extension cost?
A 3 metre kitchen extension costs £36,000 to £55,000 in total for a typical 3m x 4m or 3m x 5m footprint with a mid-range kitchen. The build alone is £24,000 to £38,000. Kitchen units and appliances add £6,000 to £15,000. The structural engineer fee is £600 to £900.
How long does a 3 metre extension take to build?
Most 3 metre single storey extensions take 8 to 12 weeks from starting on site to completion. Before work starts you need the structural engineer's calculations (typically 24 to 72 hours for the beam design) and Building Control plans approval (usually five working days).
Can I do a 3 metre extension without a builder?
You can manage some of the work yourself to save costs on finishing. However, the structural work including foundations, brickwork and steel beam installation must be carried out by an experienced builder who understands the structural drawings. An amateur installation of an RSJ beam or undersized foundations could fail inspection and require expensive remediation.
Summary: 3 Metre House Extension in 2026
A 3 metre house extension costs between £18,000 and £55,000 depending on width, finish and whether a kitchen fit-out is included. Most 3m rear extensions fall under permitted development and do not need planning permission, but Building Regulations approval and structural calculations are required for every extension without exception.
The structural engineer fee of £500 to £900 covers the RSJ beam design, foundation specification and the calculation pack Building Control needs. Getting this right at the outset avoids delays and expensive remediation during the build.
For structural calculations for your 3 metre extension, fill in the form opposite or call 07359 267907. We respond the same working day.