New Build vs Extension London 2026: Which Is Better for Your Home?

One of the most common questions LCCL Construction receives from clients is whether to extend their existing property or demolish and build a new house entirely. Both approaches have compelling advantages depending on your specific situation — and the right answer depends on your plot, budget, timeline, planning situation and long-term goals. This guide sets out an honest comparison of both options to help you make the right decision.

LCCL Construction delivers both new build houses and house extensions across London, Hertfordshire and Bedfordshire

New Build vs Extension London: The Key Differences

A new build vs house extension London adds space to your existing property — typically a new kitchen, bedroom or living space. A new build replaces the existing property entirely with a larger, purpose-designed new dwelling. The key differences are cost, timeline, planning requirements, VAT treatment, disruption and the scale of transformation each option delivers.

Cost Comparison: New Build vs Extension

House extension:

A single storey rear extension costs £35,000 – £75,000 (zero-rated for VAT).

A wraparound extension costs £60,000 – £128,000 (zero-rated for VAT).

A double storey extension costs £75,000 – £165,000 (zero-rated for VAT).

See our full house extension cost guide.

New build: Construction costs £2,200 – £4,500 per square metre (zero-rated for VAT).

A 150 square metre new build typically costs £270,000 – £525,000 (zero-rated for VAT) to construct.

See our full new build cost guide.

VAT — A Major Advantage for New Builds

This is one of the most significant financial differences between the two options. New build residential construction is zero-rated for VAT — no VAT is charged on construction costs. House extensions are subject to VAT at 20 per cent. For a £400,000 new build this means a £80,000 VAT saving compared with an equivalent renovation or extension project. This makes new builds considerably more cost-effective per square metre than extensions at scale.

Timeline Comparison: New Build vs Extension

House extension: 5 to 12 months from initial consultation to completion — significantly faster.

New build: 18 to 30 months from initial design to completion.

See our guide on how long a new build takes for a full breakdown of all stages.

Planning Permission: New Build vs Extension

House extension: Many single storey extensions can be built under permitted development without planning permission. See our house extension planning permission guide. New build: Planning permission is always required — no permitted development rights apply. See our new build planning permission guide.

Disruption: New Build vs Extension

House extension: You can often remain in your property throughout the extension — disruption is significant but manageable for most families. New build: If replacing an existing property you must vacate completely, find alternative accommodation and manage the demolition and rebuild process. This is a major practical consideration that many clients underestimate.

When Is a New Build the Better Choice?

A new build is the better choice when: the existing property is in poor structural condition and costly to repair, the plot is large enough to justify a significantly larger replacement dwelling, the VAT saving on a large construction project is financially compelling, you want a completely modern property designed from scratch to your specification, or you are developing a garden plot or infill site where no existing property stands.

When Is an Extension the Better Choice?

An extension is the better choice when: you want additional space quickly without the 18-30 month new build timeline, you want to remain in your property during works, the existing property has strong period character worth preserving, the budget is £40,000 to £200,000 rather than £300,000+, or permitted development means you can build without planning permission. See our complete guide to house extensions in London.

Frequently Asked Questions: New Build vs Extension

Is a new build cheaper than extending in London?

Not usually — a new build costs significantly more in absolute terms. However the zero-rated VAT treatment makes new builds more cost-effective per square metre at scale than extensions which are subject to 20 per cent VAT.

Can I stay in my house during a new build?

If replacing an existing property you must vacate completely. If building on a separate plot or garden you can remain in your existing property during construction.

To discuss whether a new build or extension is right for your property call LCCL Construction on 020 8037 7332 or get an instant quote online.