Extra bedroom and en-suite extensions in Mortlake
Thinking about an extra bedroom and en-suite extension in Mortlake? We design it, take it through the approvals it needs and build it, with one team responsible from the first visit to the final certificate.
Before you start in Mortlake, here's what applies.
- Postcode districts
- SW14
- County / area
- Greater London, England
- Price region
- London
- Guide price
- £67,000–£90,500 (£2,400–£3,240 per m²), plus £5,500–£11,000 for the en-suite
- Approval route
- It may be permitted development if it goes no more than 3 m beyond the original rear wall (counting any single-storey part) and stays 7 m from the rear boundary; otherwise it needs planning permission.
- Nearest towns
- East Sheen (0.5 miles), Barnes (0.9 miles), Kew (1.2 miles)
- First visit
- Free, no obligation
About an extra bedroom and en-suite extension
An extra bedroom and en-suite extension is a two-storey extension planned around a new double bedroom with its own shower room on the first floor, with a ground-floor room below. It is one of the most effective ways to add space for a growing family. Nationwide's 2025 research linked adding a double bedroom and bathroom to a three-bed, one-bathroom house with value gains of up to 24%.
We cover postcode district SW14 in and around Mortlake, along with neighbouring East Sheen, Barnes and Kew.
What it costs in Mortlake
A guide price for Mortlake: around £67,000–£90,500 (plus £5,500–£11,000 for the en-suite) for a 4 m × 3.5 m footprint on two floors (28 m² of new floor space). That works out at £2,400–£3,240 per m² of new floor space, including VAT at 20%. Professional fees for drawings and the structural engineer are listed separately on our prices page.
| Spec | Per m² | A 4 m × 3.5 m footprint on two floors (28 m² of new floor space) |
|---|---|---|
| Essential | £2,210–£2,980 per m² | £62,000–£83,500 |
| Standard | £2,400–£3,240 per m² | £67,000–£90,500 |
| Premium | £2,880–£3,890 per m² | £80,500–£109,000 |
Planning and building regulations in Mortlake
In England, an extra bedroom extension at the rear can be permitted development if it goes no more than 3 m beyond the original rear wall and stays 7 m from the rear boundary. Upper side windows, including en-suite windows, must be obscure-glazed and non-opening below 1.7 m. Side or deeper designs need planning permission, costing £548 (or £575 for applications made from 8 December 2026).
Approval in England: Householder planning application: £548 (or £575 for applications made from 8 December 2026). Lawful development certificate: £274 (about £288 for applications from 8 December 2026). Councils should decide householder applications within 8 weeks of validation.
Building regulations: Building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end.
Design points for Mortlake homes
Sound and privacy
Bedrooms above a kitchen or family room benefit from an insulated floor to reduce noise. Side-facing upstairs windows usually have to be obscure-glazed, so we place the main bedroom window to the rear and the en-suite window to the side where possible.
Planning the en-suite
An en-suite needs a route for the soil pipe to the drains, enough fall for waste water and an extract fan to the outside under Part F. Placing it over the ground-floor utility or near an existing drain keeps pipework short and hidden.
How we'd run your project
Free site visit
We visit your home in Mortlake, look at the structure, access and boundaries, and talk through what you want the new rooms to do.
Plans, then approval
Measured survey, design, then the right approval route for your house. We explain each step before we take it.
Technical design
We prepare the building regulations drawings, structural calculations and specification so the build price can be fixed.
Finish and certify
Second fix, plastering and making good, then building control's final inspection and the completion paperwork.
Questions about extra bedroom and en-suite extensions in Mortlake
Do I need planning permission for an extra bedroom and en-suite extension in Mortlake?
Mortlake is in England, so England planning rules apply. It may be permitted development if it goes no more than 3 m beyond the original rear wall (counting any single-storey part) and stays 7 m from the rear boundary; otherwise it needs planning permission. If in doubt, a lawful development certificate or a planning application gives you certainty before you build.
Which postcodes do you cover around Mortlake?
We cover Mortlake (SW14) and nearby places including East Sheen, Barnes and Kew, plus the rest of Greater London. The first site visit is free.
How long does an extra bedroom and en-suite extension take?
Allow 4 to 8 weeks for survey and design, then the approval period (councils should decide householder applications within 8 weeks of validation), then 4 to 8 weeks of technical design, which can overlap. The build itself usually takes 14 to 20 weeks, longer for large or complex designs.
Does an extra bedroom add value to my house?
It usually does, though not always by the full cost. Nationwide's 2025 research found that adding a double bedroom to a two-bed house was linked to about 13% higher value, and a double bedroom and bathroom on a three-bed, one-bathroom house up to 24%. Local ceiling prices still apply.
What goes on the ground floor below a new bedroom?
Whatever suits you: a bigger kitchen, a family room, a study, a utility or a downstairs shower room. The ground floor usually costs about the same whatever it is used for, apart from kitchen units and bathroom fittings, which are priced separately.
Nearby
We also cover places near Mortlake, including:
Other extensions in Mortlake
All two-storey extension services in Mortlake · Extra bedroom and en-suite extensions across Greater London
Book a free site visit in Mortlake and we'll tell you what's possible, what approvals you need and what the design stage would cost.