Two-storey extensions for detached houses in Croxley Green

The basics for Croxley Green at a glance.

Postcode districts: WD3
County / area: Hertfordshire, England
Price region: East of England
Guide price: £88,500–£119,000 (£2,110–£2,830 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: Batchworth (0.6 miles), Rickmansworth (1.2 miles), Holywell (Hertfordshire) (1.4 miles)
Distance to Watford: about 2.8 miles
First visit: Free, no obligation

About a two-storey extension for a detached house

A two-storey extension for a detached house is a two-floor addition to the side, rear or both of a house that stands on its own plot. Detached houses often have room on more than one side, so there is more choice of layout and fewer shared-wall issues. The design still needs to respect boundaries, neighbours' windows and the look of the house.

Croxley Green sits about 2.8 miles from Watford. Closer to home, Batchworth is 0.6 miles away and Rickmansworth 1.2 miles.

What it costs in Croxley Green

For budgeting in Croxley Green, allow around £88,500–£119,000 for a two-storey extension for a detached house with a 3 m × 7 m footprint on two floors (42 m² of new floor space). That's based on £2,110–£2,830 per m² including VAT for the East of England. Keep a contingency of around 10% for things nobody can see until the ground is opened up.

Spec	Per m²	A 3 m × 7 m footprint on two floors (42 m² of new floor space)
Essential	£1,940–£2,610 per m²	£81,500–£109,500
Standard	£2,110–£2,830 per m²	£88,500–£119,000
Premium	£2,530–£3,400 per m²	£106,500–£143,000

Planning and building regulations in Croxley Green

In England, a two-storey rear extension on a detached house can be permitted development within 3 m of the original rear wall and at least 7 m from the rear boundary, with matching roof pitch and materials, outside protected areas. Any two-storey side or wraparound extension needs planning permission, costing £548 (or £575 for applications made from 8 December 2026), with a decision due within 8 weeks of validation.

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 Croxley Green homes

Larger spans and steelwork

Bigger extensions often mean wider openings and longer spans, which need more steel and sometimes a steel frame. The engineer designs these early, because the size of the steel affects ceiling heights, floor depths and the cost.

Keeping it subordinate

Large detached houses can take a large extension, but councils still expect it to look secondary to the main house. A lower ridge, a modest set-back at the front and matching materials keep the proportions right, even on a wide plot.

How we'd run your project

Look at the house
We start in Croxley Green with a free visit: measurements, photos, a look at the loft and drains, and a frank chat about budget.
Choose the approval route
Some designs fit permitted development, most need a householder application. We decide which applies in England before drawing in detail.
Structure and building control
A structural engineer sizes the foundations and steels, and we get building regulations approval before work starts.
Build and hand over
We manage the build from groundworks to decoration-ready finish and hand over with the completion paperwork.

Questions about two-storey extensions for detached houses in Croxley Green

Do I need planning permission for a two-storey extension for a detached house in Croxley Green?

Croxley Green 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.

Which postcodes do you cover around Croxley Green?

We cover Croxley Green (WD3) and nearby places including Batchworth, Rickmansworth and Holywell (Hertfordshire), plus the rest of Hertfordshire. The first site visit is free.

How much does a two-storey extension for a detached house cost in Croxley Green?

Our guide for the East of England, which covers Croxley Green, is £2,110–£2,830 per m² including VAT. A typical project with a 3 m × 7 m footprint on two floors (42 m² of new floor space) comes to about £88,500–£119,000. Design and approval fees are extra, and we fix the build price in writing once the design is agreed.

Is a detached house extension a better investment than moving?

It often costs less than the gap between your house and a larger one nearby, once property tax, legal fees and removal costs are counted, but that depends on your area. Extending also has limits: every street has a ceiling price. We give you build costs so you can compare them with local asking prices.

How big an extension can a detached house take?

It depends on the plot, the boundaries and local design policy rather than a fixed limit. Under permitted development in England, extensions must not cover more than 50% of the land around the original house. With planning permission, the council judges size on proportion, neighbours and the character of the area.

Nearby

Nearby towns we cover for two-storey extensions for detached houses:

Other extensions in Croxley Green

All two-storey extension services in Croxley Green · Two-storey extensions for detached houses across Hertfordshire

Book a free site visit in Croxley Green and we'll tell you what's possible, what approvals you need and what the design stage would cost.