Welwyn Garden City, Hertfordshire
Semi-detached rear extension in Welwyn Garden City
Single or two-storey extensions for semi-detached houses, with side access and one party wall. If you live in Welwyn Garden City and want to know what a semi-detached rear extension would cost and whether it needs planning, start with the online guide price or a free site visit.
Welwyn Garden City is covered from our Hertfordshire round, including AL7 and AL8. In Welwyn Garden City we measure the attached neighbour's extension at the survey, because it often sets the sensible depth for yours.
Book a free site visit
Tell us about the house and we'll call to arrange a free, no-obligation visit.
How much does a semi-detached rear extension cost in Welwyn Garden City?
Guide prices for a semi-detached rear extension in Welwyn Garden City start from our East of England rate of £2,010–£3,290 per square metre, including VAT. East of England prices run slightly above our West Midlands baseline; clay soils near trees are the main foundation variable.
| Size | Guide price inc VAT |
|---|---|
| 3 m wide × 4 m deep (12 m²) | £24,000–£39,500 |
| 4 m wide × 5 m deep (20 m²) | £40,500–£65,500 |
| 4 m wide × 6 m deep (24 m²) | £48,500–£79,000 |
Your written price follows a measured survey and an agreed design, so it reflects your house, not an average.
Do you need planning permission for a semi-detached rear extension in Welwyn Garden City?
The planning route for a semi-detached rear extension in Welwyn Garden City is set by England's householder rules and your council.
Under England's householder permitted development rules you can usually add 3 metres at the back of an attached house, or 4 metres on a detached house, without a planning application, as long as the extension stays under 4 metres high and its eaves stay under 3 metres where it is within 2 metres of a boundary.
England's rules allow a rear extension of more than one storey without an application only when it is no deeper than 3 metres and at least 7 metres from the boundary opposite the rear wall, and never on designated land. Most two-storey schemes need householder planning permission.
If 3 or 4 metres is not enough, England's prior approval route allows 6 metres on a terraced or semi-detached house and 8 metres on a detached house for a single-storey rear extension. Neighbours are consulted, the council decides within 42 days, and the fee is £249, rising to £310 from 8 December 2026.
Conservation areas, National Parks, National Landscapes and the Broads restrict permitted development, and listed buildings always need listed building consent. At the first visit we measure what is there now and tell you which route fits: permitted development, prior approval or a planning application.
What are homes in Welwyn Garden City like to extend?
Welwyn Garden City mixes Victorian terraces with 1930s semis, so both side returns and full-width kitchen extensions suit houses here.
How a semi-detached rear extension in Welwyn Garden City comes together
Free site visit
We measure the house and garden, look at the back wall, drainage and boundaries, and talk through what you want from the space.
Drawings and budget
Floor plans and elevations for the options, each with a guide price.
Paperwork handled
Planning, prior approval or permitted development, Building Regulations approval, structural design and party wall notices are all handled by us.
Fixed price and build
A written price for the agreed design, a start date and a named contact through the build, ending with the building control completion certificate.
Semi-detached rear extensions near Welwyn Garden City
Neighbouring places we cover include Hatfield (2.8 miles) and Codicote (3.6 miles).
- Semi-detached rear extension in Hatfield
- Semi-detached rear extension in Codicote
- Semi-detached rear extension in Hertford
- Semi-detached rear extension in St Albans
- Semi-detached rear extension in Harpenden
Other work in Welwyn Garden City:
FAQ
Semi-detached rear extension questions in Welwyn Garden City
Do you handle building control in Welwyn Garden City?
Yes. We apply to the council or a registered building control approver and hand over the completion certificate.
Can you match the brick or stone on my Welwyn Garden City house?
We source the closest match and show you samples before ordering; reclaimed materials are often the best match on older houses.
How long does a semi extension take?
10 to 16 weeks on site for a single-storey extension.
Can I get a price without a visit?
Our online guide-price tool gives a range in a couple of minutes. A written price needs a survey.
Is the site visit in Welwyn Garden City free?
Yes. We visit, measure up and talk through the options with no cost and no obligation.
Do I need planning permission for a rear extension in Welwyn Garden City?
Often not for a single-storey rear extension of up to 3 metres (4 metres on a detached house) under permitted development, or up to 6 or 8 metres with prior approval, unless the house is in a conservation area or its rights have been removed.