Ask three builders what a single storey extension costs and you will get three different answers, because the number depends far more on what goes into the box than the size of the box itself. This guide sets out realistic figures for Hertfordshire in 2026, explains where the money actually goes, and flags the costs homeowners routinely forget until the quotes land.
As a working figure, most single storey rear extensions in south Hertfordshire land between £2,200 and £3,200 per square metre for the build itself, excluding VAT, kitchens and furniture. A straightforward 20 square metre rear extension with a pitched or flat roof, two windows and a set of French doors therefore tends to sit somewhere around £44,000 to £64,000.
Push the specification up and the rate climbs quickly. Large aluminium sliding doors, structural glazing, a full-width opening into the existing house needing a steel beam, underfloor heating and a high end kitchen can take a similar footprint past £90,000. Costs in Borehamwood, Radlett and Elstree tend to sit slightly above the national average because of London labour rates and access constraints on tighter plots.
Groundworks are the biggest unknown. Older properties around Borehamwood and Bushey often sit near mature trees, and clay soil in this part of Hertfordshire means foundations may need to go far deeper than the standard one metre. A tree protected by a preservation order or a neighbouring root system can add £3,000 to £8,000 in extra concrete and labour before anything above ground exists.
Drainage is the second surprise. If a foul or surface water run crosses your extension footprint, you will need a build over agreement with Thames Water, plus possible diversion or a new inspection chamber. Allow around £400 for the application and anything from £1,500 upwards if the run has to move.
Access matters more than people expect. A terraced house with no side gate means every barrow of muck and every bag of ballast travels through the hallway, which slows the job and adds labour days. A detached property with driveway access for a grab lorry can be noticeably cheaper to build on for the same floor area.
Budget realistically for the professional fees before you commit. Architectural drawings for a modest extension typically run £1,500 to £3,500, structural engineer calculations £500 to £1,200, and a planning application fee to Hertsmere Borough Council currently sits at £258 for a householder application. Building control, whether through Hertsmere or an approved inspector, is usually £700 to £1,200.
If you share a wall or are excavating within three metres of a neighbour's foundations, the Party Wall etc. Act 1996 applies. Serve notice yourself and it costs nothing but time. If a neighbour dissents, surveyor fees commonly reach £1,200 to £2,500 and you pay them.
Many single storey rear extensions in Hertfordshire fall under permitted development rather than full planning, which saves time but still requires a lawful development certificate if you want proof for a future sale. Conservation areas in places like Aldenham and parts of Elstree remove some of those rights entirely.
A cheap quote is usually a thin quote. Ask every builder to price the same specification and to state what is excluded, because the difference between £48,000 and £62,000 is frequently just whether the kitchen, flooring, decoration and steelwork are inside the figure or not.
Insist on a written payment schedule tied to stages rather than dates, and hold a retention of around five per cent for six months after completion to cover snagging. Any request for a large deposit before work starts is worth questioning; a modest sum to secure materials is normal, half the contract value is not.
Most single storey extensions take 10 to 16 weeks on site once work begins, with the shell watertight around week six. Planning and drawings usually add another two to four months before that.
In parts of Hertsmere, a well built extension that adds a proper kitchen diner often returns most of its cost, but rarely all of it in a flat market. Build it because you want the space, and treat any uplift in value as a bonus.
Many single storey rear extensions fall within permitted development, typically up to three metres on an attached house or four metres on a detached one, with larger projects possible under the prior approval route. Conservation areas, flats and previously extended properties are the common exceptions, so check with Hertsmere before assuming.
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