House Extensions Borehamwood

GS Renovation builds extensions in Borehamwood, from groundworks and steel through to roofing, glazing and the final fit out. We appoint the structural engineer, coordinate the planning drawings with your architect or designer, and build to what Hertsmere Borough Council approves. Building control applications and inspections are coordinated as part of the same job. For a single storey rear kitchen extension on a Borehamwood semi or terrace, the depth, height and garden limits below decide how large the new kitchen can be, and the clay decides how deep it is founded.

For Borehamwood, 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 Borehamwood: the structural engineer we appoint designs the structure, we build it, and building control signs it off.

The ground a Borehamwood extension is founded on

London Clay. British Geological Survey 1:50,000 mapping records the London Clay Formation as the bedrock beneath Borehamwood. BGS classifies it as part of the Thames Group, deposited in the Ypresian Age of the Eocene, and describes it as clay, silt and sand. Clay changes volume with moisture, which is why foundation depth on a clay site is an engineering decision rather than a standard detail copied from the last job.

The practical consequence is trees. Where a mature tree sits within influencing distance of the new footprint, the engineer looks at species and distance before setting the depth, and the excavation can go deeper than a first guess. That is a cost worth establishing early rather than discovering in a trench. We dig trial holes where the engineer asks for them instead of pricing a foundation blind.

Housing in Borehamwood

Borehamwood has about 15,090 homes on the Council Tax list: 2% pre-1919, 8% interwar and 90% post-war, with flats making up 36% and houses 63%. A rear extension on a terrace in Borehamwood digs foundations close to both neighbours, so notices to both are normal, and 38% of homes here are terraced.

How deep and how tall Hertsmere expects an extension to be

Hertsmere states it is likely to refuse a rear extension that breaches the 45 degree line or extends more than 3.35 metres beyond the original rear building line. That 45 degree line is drawn from the nearest edge of a neighbour's habitable room or kitchen window, and every extension is expected to sit comfortably within it. Only habitable room and kitchen windows are taken into consideration.

On height, Hertsmere expects a single storey extension to be no more than 3 metres high with a flat roof, or 4 metres with a pitched roof, measured from ground level. Those two figures decide whether a pitched roof is worth having on a given plot at all. We check both against your actual levels before drawings go in, because a scheme that misses the limit is a scheme drawn twice.

The garden Hertsmere expects you to keep

It scales with bedrooms. Hertsmere's guide expects a two or three bedroom house to retain 60 square metres of rear garden after an extension. A four bedroom house is expected to retain 80 square metres, and a house of five or more bedrooms 100 square metres. Measure the garden before deciding how far back to build, because that figure can bind before the 3.35 metres does.

Plot type matters here too. The 2021 Census recorded 740 detached households across the four Borehamwood wards, out of 14,197 households in total, and in Cowley Hill ward the figure was 144 detached out of 3,725 households. An attached or semi detached house brings a neighbour's windows into the 45 degree calculation on at least one side, so the garden test and the daylight test can bind together.

Conservation areas and the levy in Borehamwood

There is no conservation area in Borehamwood. Hertsmere states that there are 15 conservation areas in the borough, and none of them is in the town. The nearest is Elstree Village, designated on 4 August 1976. That removes one whole layer of constraint from an extension here, which is not the case everywhere in Hertsmere.

On the Community Infrastructure Levy, Borehamwood sits in Hertsmere's Area A together with Elstree and Potters Bar. For permissions granted during 2026 the indexed residential rate is £200.84 per square metre, against £301.26 in Area B, which covers Bushey and Radlett. Residential development in the Elstree Way Corridor carries a rate of £0 per square metre. Whether a charge arises on your particular scheme is Hertsmere's decision.

Kitchen extensions in Borehamwood

60% of the houses in Borehamwood are terraced. On a terrace the kitchen extension is built out from the back, into the side return where the house has one or across the full rear width, and a steel beam takes the old back wall so the kitchen and the new room become one space. The fit-out is priced on its own page, kitchen renovation in Borehamwood; the build figures are in the table above.

Side return extensions in Borehamwood

Side returns belong to Victorian and Edwardian terraces, and only 2% of homes in Borehamwood 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 Borehamwood

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 Borehamwood often needs planning permission from Hertsmere Borough Council. On the 38% of homes that are terraced, the neighbours' windows usually decide the depth. Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.

• Foundations, groundworks and steel frames
• Party wall process explained, and the extension built to the award
• Drawings and the structural engineer's design coordinated for you

VOA Council Tax stock of properties, March 2025, for Borehamwood Cowley Hill, Borehamwood Brookmeadow, Borehamwood Kenilworth and Borehamwood Hillside. Counts rounded by the VOA.

Two or three things usually come up alongside house extensions on a Borehamwood job: RSJ and steel beam installation in Borehamwood, house extensions in Radlett, wraparound extensions, the Party Wall Act process for extensions, house extensions across London and Crittall Doors Guide.

Why Borehamwood Homeowners Choose GS Renovation

An extension in Borehamwood is a planning conversation with Hertsmere before it is a building job. We coordinate the drawings with your designer, establish the design guide position on depth, height and retained garden, appoint the structural engineer and build the whole thing with our own trades. Building control applications and inspections coordinated, fully insured, free no obligation quotations.

• Foundation depth set after trial holes, drains and trees are checked
• Garden levels and access made good where the build meets outside
• A written scope and programme agreed before work starts

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.