GS Renovation designs and builds extensions in Borehamwood, from groundworks and steel through to roofing, glazing and the final fit out. We appoint the structural engineer, prepare the drawings and deal with Hertsmere Borough Council on the planning route. Building control applications and inspections are coordinated as part of the same job.
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.
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.
- Party Wall Act notices for excavation near a neighbour's wall
- Foundations, groundworks and steel frames
- Flat and pitched roofs with rooflights or lanterns
- Side-return and rear extensions
- Insulation, airtightness and thermal detailing to regulations
Why Borehamwood Homeowners Choose GS Renovation
An extension in Borehamwood is a planning conversation with. Hertsmere before it is a building job. We prepare the drawings, 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.
- Registered in England, Companies House no. 14611254
- Party Wall notices served for excavation near a neighbour's wall
- Materials priced at options, so you choose the finish
- 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