Two-storey extensions for semi-detached houses in Collier Row

Useful details for planning a two-storey extension for a semi-detached house in Collier Row.

Postcode districts: RM5
County / area: Greater London, England
Price region: London
Guide price: £67,000–£90,500 (£2,400–£3,240 per m²)
Approval route: Depends on position: a two-storey rear extension may be permitted development (3 m deep, 7 m from the rear boundary); a two-storey side extension needs planning permission.
Nearest towns: Romford (1.8 miles), Chadwell Heath (2.4 miles), Harold Wood (2.9 miles)
First visit: Free, no obligation

About a two-storey extension for a semi-detached house

A two-storey extension for a semi-detached house is a two-floor addition to the rear, the open side, or both, designed around the attached neighbour on one side. The rear is often the simplest option and can sometimes be permitted development, while a side extension uses the gap on the detached side. Both need care with the shared party wall and the neighbour's light.

Around Collier Row we also work in Romford, Chadwell Heath and Harold Wood, and across the rest of Greater London. Postcode district in and near Collier Row: RM5.

What it costs in Collier Row

Using our London rates, a two-storey extension for a semi-detached house with a 4 m × 3.5 m footprint on two floors (28 m² of new floor space) comes to about £67,000–£90,500, including VAT. Bigger or more glazed designs cost more per m². Keep a contingency of around 10% for things nobody can see until the ground is opened up.

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 Collier Row

In England, a two-storey rear extension on a semi 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, with matching materials and roof pitch. A two-storey side or wraparound extension always needs planning permission, costing £548 (or £575 for applications made from 8 December 2026). Protected areas such as conservation areas exclude two-storey rear permitted development.

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 Collier Row homes

Rear, side or both

On a semi, a rear extension sits against the attached neighbour, while a side extension faces the detached neighbour and the street. We compare the space, cost and approval route for each, and for a wraparound, before you commit to one design.

Keeping the pair balanced

Semis are designed as matching pairs, and a side extension changes that balance. Setting the side extension back from the front wall and keeping its ridge lower helps the pair still read as a pair, which councils' design guides commonly ask for.

How we'd run your project

Look at the house
We start in Collier Row with a free visit: measurements, photos, a look at the loft and drains, and a frank chat about budget.
Drawings and permission
Our designers produce plans and elevations, then we take care of planning or permitted development checks with the council covering Collier Row.
Structure and building control
A structural engineer sizes the foundations and steels, and we get building regulations approval before work starts.
Finish and certify
Second fix, plastering and making good, then building control's final inspection and the completion paperwork.

Questions about two-storey extensions for semi-detached houses in Collier Row

Do I need planning permission for a two-storey extension for a semi-detached house in Collier Row?

Collier Row is in England, so England planning rules apply. Depends on position: a two-storey rear extension may be permitted development (3 m deep, 7 m from the rear boundary); a two-storey side extension needs planning permission. If in doubt, a lawful development certificate or a planning application gives you certainty before you build.

How long does a two-storey extension for a semi-detached house 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.

Who handles building regulations for a project in Collier Row?

We handle it as part of the project. What's needed in England: building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end. We prepare the drawings and the engineer's calculations, book the inspections and pass you the completion paperwork at the end.

Does a two-storey extension add value to a semi-detached house?

It usually adds value, but the gain is never certain. Nationwide's 2025 research found that 10% more floor area was linked to about 5% higher value, and that adding a double bedroom and bathroom to a three-bed, one-bathroom house could add up to 24%. Every street has a ceiling price, so check local sales first.

What if my neighbour has already extended?

That often helps. An existing extension next door can reduce the effect of yours on their light, and may show what the council accepts on your street. It can also mean building alongside their wall, so we check its position, foundations and any windows facing your plot at the survey.

Nearby

The same service in neighbouring towns:

Other extensions in Collier Row

All two-storey extension services in Collier Row · Two-storey extensions for semi-detached houses across Greater London

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