House Extensions Radlett

An extension in Radlett starts with the ground, because the geology here is not what the rest of south Hertfordshire leads people to expect. We appoint the structural engineer, build to the foundation design that comes back, and coordinate the building control inspections from trench bottom to completion.

For Radlett, on London-derived ranges a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m²; Hertfordshire work generally runs below that. GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Radlett: the structural engineer we appoint designs the structure, we build it, and building control signs it off.

The ground under a Radlett extension is not London Clay

British Geological Survey mapping places the London Clay Formation in the low ground south of Radlett and at the southern and eastern fringes of the built up area. That includes points on the south end of Watling Street, Aldenham Road and Theobald Street, but not beneath the main body of the village, where chalk and Lambeth Group are mapped. The London Clay assumptions used across much of north London therefore do not automatically apply here. The made Radlett Plan, for its part, describes the village's underlying geology as mainly chalk.

BGS 1:50,000 mapping shows the bedrock beneath Radlett station and the northern part of the Watling Street frontage as Lewes Nodular Chalk Formation and Seaford Chalk Formation, undifferentiated. Further south through the village centre the mapped unit changes to Seaford Chalk and Newhaven Chalk. The bedrock also alternates between chalk and Lambeth Group clay, silt and sand over short distances, sometimes within a single street. BGS sampling along Watling Street returned four different bedrock units, and Craigweil Avenue and Cobden Hill each split along their own length.

Housing in Radlett

Radlett's housing, as the Council Tax list records it, is about 3,500 homes: 76% houses, 24% flats, with 12% pre-1919, 19% interwar and 67% built since 1945. A detached house in Radlett often extends far enough from the boundary that no party wall notice is needed, and 47% of homes here are detached.

What that means street by street

It means a neighbour's foundation design is a poor guide to yours. BGS mapping shows the bedrock under Gills Hill Lane, Williams Way and Loom Lane as Lambeth Group, described as clay, silt and sand, rather than chalk. Goodyers Avenue and Oakridge Avenue, by contrast, are mapped over chalk, and Park Road is split between the two.

Above the bedrock the picture changes again. BGS records superficial deposits of the Gerrards Cross Gravel Member, described as sand and gravel, over parts of Radlett. Those include Gills Hill Lane, Loom Lane, Aldenham Avenue, Christchurch Crescent and points in the Watling Street village centre. At sampled points on Newberries Avenue, Craigweil Avenue, Links Drive, The Close, Goodyers Avenue and Oakridge Avenue it records none, which places bedrock at or close to the surface. The engineer we appoint sets your depth from trial holes on your plot, not from the road name.

Extending a bungalow in one of the Article 4 groups

Whether you can go upwards depends on which schedule your address sits on, because Hertsmere's Article 4 Directions over named Radlett bungalows remove the right to add storeys. The 2015 buildings survey for the Radlett neighbourhood plan recorded 108 bungalows, 4.1 per cent of the stock, and 63 chalet bungalows, 2.4 per cent. A separate Article 4 Direction covering Group 5, numbers 35 and 37 Gills Hill Lane, was confirmed on 25 May 2023 and modified by the Secretary of State on 14 June 2023.

The bungalow Directions also remove Part 20, Class AC and Part 20, Class AD. Those are the rights to create new residential units on top of an existing terraced or semi detached house, and on top of an existing detached house. Going out at the back and going up are therefore two separate questions with two separate answers. We scope them separately at survey so a scheme is not drawn around a right that has been withdrawn.

Green Belt, trees and the brook at the boundary

Radlett is surrounded by Metropolitan Green Belt, consisting of arable farmland, grassland, woods and recreational facilities. Hertsmere Borough Council also holds separate Article 4 Directions covering Little Kendals Farm on Watling Street, land at Kendal Hall Farm and land at Kendal Wood. A plot on the edge of the village therefore behaves differently from one in the middle of it, and the difference shows up in what is permitted before design begins.

Water and trees set the other two boundaries. The Environment Agency classifies the Radlett Brook, which runs through Radlett along with its tributaries, as a main river. It has recommended a naturalised buffer zone of at least 9 metres from the top of the bank for any scheme beside the brook or its tributaries. That is in line with Hertsmere's 2016 adopted Local Plan policy SADM16, Watercourses, and the Radlett Plan carries it through as its own Policy HD6, Tykes Water. The Radlett Plan also notes that the council's online mapping of Tree Preservation Orders is understood to be out of date, and asks applicants to clarify the status of trees with the tree officer.

Kitchen extensions in Radlett

83% of the houses in Radlett are semi-detached or detached, which usually leaves room for a kitchen extension across the full rear width, with a steel beam over the opening into the house. The fit-out is priced on its own page, kitchen renovation in Radlett; the build figures are in the table above.

Side return extensions in Radlett

Side returns belong to Victorian and Edwardian terraces, and only 12% of homes in Radlett predate 1919, so they are the exception here; most extensions run across the full rear width instead. Our side return extension page covers the build in detail, with build rates of £2,400 to £3,200 per m².

Double storey extensions in Radlett

Under permitted development a two-storey rear extension can go no more than 3 metres beyond the original rear wall and must stay at least 7 metres from the rear boundary, and in a conservation area it is not permitted development at all. Many gardens are too short for the 7 metre rule, so a double storey extension in Radlett often needs planning permission from Hertsmere Borough Council. With 63% of homes here semi-detached or detached, the side of the house is often an option too. Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.

• Full fit-out, finishing and making good
• Side-return and rear extensions
• Building control inspections at foundation, damp course and steel stages

VOA Council Tax stock of properties, March 2025, for Radlett. Counts rounded by the VOA.

If this is your first project in Radlett, these are worth a look: RSJ and steel beam installation in Radlett, house extensions in Borehamwood, double storey extensions, party walls and extensions, how we handle house extensions across London and Side Return vs Rear Extension.

Why Choose GS Renovation in Radlett

An extension in Radlett is won or lost below ground, and that is where we put the effort first. Trial holes and the engineer's foundation design come before the drawings are finalised, because the bedrock under this village changes within a street and the depth you dig sets the cost of everything above it. From there one team carries the shell, the steelwork, the roof and the internal openings. We are fully insured, building control applications and inspections are coordinated for you, and quotations are free and carry no obligation.

• Foundation depth set after trial holes, drains and trees are checked
• Insulation and thermal detailing to current regulations
• Waste cleared as we go, not left piled up outside

Checked by Abdelaaziz Saaid, founder and director of GS Renovation Ltd (Companies House no. 14611254). Structural design is by the consulting engineer appointed to your project.