Single Storey Extension Costs in Hertfordshire: A 2026 Guide

What you can expect to pay in 2026

In Hertfordshire, most single storey extensions land between roughly £2,200 and £3,200 per square metre for the build itself, before VAT, fees and finishes. Because we sit just outside Greater London, labour and material delivery costs tend to run a little higher than the national average quoted by sites covering the Midlands or the North.

As a rough guide for a standard rear extension to a good but not luxury specification:

• Small (around 15m²): roughly £40,000 to £58,000
• Medium (around 25m²): roughly £58,000 to £85,000
• Large (around 40m²): roughly £90,000 to £130,000
• Add VAT at 20% on top, as most home extensions are not zero-rated

What actually moves the price

Two extensions of identical size can differ by £20,000 or more, and it usually comes down to the same handful of factors. Knowing them early helps you set a sensible budget rather than chasing the cheapest quote.

Ground conditions matter more than people expect. Clay soil is common across Borehamwood and much of south Hertfordshire, and nearby trees can mean deeper foundations or a piled solution, which adds cost before you have built a single wall.

• Glazing: large sliding or bi-fold doors and roof lanterns add several thousand pounds
• Kitchen or bathroom fit-out, where plumbing and units sit inside the new space
• Steel beams to open up the rear of the house into one room
• Roof type, with a flat roof generally cheaper than a pitched or vaulted one
• Finish level, from a sensible builder's spec to high-end fittings

Planning and permitted development locally

Many single storey rear extensions fall under permitted development, meaning you may not need full planning permission. Limits depend on whether your home is detached, semi or terraced, and on how far you extend from the original rear wall.

That said, parts of Hertfordshire include conservation areas and properties with article 4 directions or restrictive covenants, where permitted development rights are reduced or removed. It is always worth a quick check with Hertsmere Borough Council, or your relevant local authority, before committing. Budget for building control fees and, where needed, a structural engineer and party wall agreement if you share a boundary.

Where the money is well spent

The cheapest quote is rarely the best value. The things that protect your investment are proper foundations, good insulation to meet current building regulations, and quality waterproofing on a flat roof, none of which are visible once finished but all of which cause expensive problems if skimped.

Ask any builder for a written, itemised quote rather than a single lump sum. It lets you compare like for like, see where the money goes, and spot whether items such as making good, decoration and waste removal are included or quietly left out.