Ask three builders what a single storey extension costs and you will get three very different answers, usually because they are quoting three different specifications. The honest answer for Watford and the surrounding Hertfordshire area sits somewhere between £1,900 and £3,000 per square metre for the build itself, and where you land inside that range depends on decisions you make long before the first brick is laid. Here is what actually drives the number.
A typical 4m x 5m rear extension in Watford, so 20 square metres, tends to come in around £42,000 to £58,000 for a standard specification finished to plaster. That covers groundworks, foundations, the brick and block shell, roof structure and covering, windows and external doors, first fix electrics and plumbing, insulation and plasterboard.
It does not usually cover kitchen units and appliances, flooring, decorating, bifold upgrades, underfloor heating, or landscaping to make good the garden afterwards. Those items regularly add £8,000 to £25,000 and are the single biggest reason people feel a quote has 'gone up' when it has not.
Ground conditions matter more here than people expect. Large parts of Watford, Bushey and Croxley Green sit on London Clay with mature trees nearby, and building control will often require deeper foundations or a heave precaution such as Cellcore. Going from a 1m trench fill to 2.5m on a modest extension can add £4,000 to £9,000 on its own, and it is invisible once the job is done.
Access is the other Watford factor. Terraced and semi-detached streets around Callowland, Nascot Wood and West Watford often have no side access, meaning every barrow of muck and every pack of bricks goes through the house. Expect that to add labour time and protection costs. Skip permits from Hertfordshire County Council for on-street placement are a small but real line item too.
Matching existing brickwork is worth budgeting for properly. Much of Watford's interwar housing uses stocks or reds that are no longer in production, so a reclaimed or specially sourced match can cost two to three times a standard facing brick. It is money well spent, because a mismatched rear elevation is the thing people notice for years.
The cheapest square metre is a simple rectangle with a flat roof, a standard set of French or sliding doors and a straightforward internal layout. Every corner, every step in the roofline and every structural opening into the existing house adds steel, labour and time.
Keeping the existing kitchen and bathroom drainage runs roughly where they are avoids moving inspection chambers, which is a genuinely expensive job. Similarly, taking out a 3m section of rear wall rather than the full width can halve the steel and needling costs while still giving you the open feel.
Beyond the builder, budget for structural engineer calculations at £500 to £1,200, building control at roughly £900 to £1,500 through Watford Borough Council or an approved inspector, and drawings at £1,200 to £2,500. If your extension sits on or near a boundary with a neighbour, a Party Wall award will typically cost £1,000 to £2,000 per adjoining owner.
Most single storey rear extensions in Watford fall under permitted development, but conservation areas and Article 4 restrictions apply in parts of the borough, and a full planning application changes both the timeline and the cost. Check before you budget, not after.
A straightforward 20 square metre extension usually takes 10 to 14 weeks on site, assuming reasonable weather and no surprises in the ground. Kitchen fitting and decorating typically add another two to four weeks afterwards.
Not automatically, but read what is actually included, particularly foundation depth, steel specification, glazing and whether finishes are in the price. A low quote based on assumed shallow foundations will not stay low once building control inspects the trench.
Staged payments tied to completed work are normal and reasonable, and a modest deposit for materials is standard. Be cautious of anyone asking for a large sum before work begins or wanting payment well ahead of the stage reached.