What Does a Home Extension Really Cost in 2026? Your Honest Guide

With the long summer days here and the school holidays just around the corner, June is when a lot of homeowners in Bournemouth, Poole, and Christchurch start seriously thinking about that extension they've been putting off. Maybe you've outgrown your kitchen, need an extra bedroom, or just want to claim some more living space from the garden. Whatever the reason, the first question is almost always the same: how much is this actually going to cost?

It's a fair question and one that deserves a straight answer. So here's our honest breakdown. 

Why Extension Costs Vary So Much.

You've probably typed "home extension cost" into Google and come away more confused than when you started. That's because the range really is wide and for good reason. A single-storey rear extension on a terraced house in Poole is a very different project to a two-storey side extension on a detached property in the New Forest. Size, specification, ground conditions, and finish all play a huge role.

That said, having some ballpark figures to work with is genuinely useful when you're in the planning stages.

Typical Extension Costs in 2026.

Here's a general guide to what you might expect to pay for common extension types in our area:

Single-Storey Rear Extension

• Small (up to 15m²): £25,000–£40,000
• Medium (15–30m²): £40,000–£65,000

These are among the most popular types of home extension we carry out in Bournemouth and Poole, ideal for opening up a kitchen-diner or creating a larger family room.

Two-Storey Extension

• Typical range: £65,000–£120,000+

Two-storey builds offer much better value per square metre than single-storey, as you're sharing the cost of the foundations, roof, and scaffolding across two floors. If you need both extra ground floor and bedroom space, this is often the smarter long-term investment.

Side-Return Extension 

• Typical range: £30,000–£55,000

Very common in older terraced and semi-detached properties across Christchurch and Bournemouth, where that narrow side passage gets transformed into a proper part of the home.

What Affects the Final Price?

Beyond square footage, a few key factors will shape your quote:

• Specification and finishes — Bi-fold doors, underfloor heating, and high-end kitchen or bathroom fittings all add up. Be clear with your builder about what's included.
• Structural complexity — Removing load-bearing walls or working around awkward ground conditions (not uncommon close to the coast) adds cost.
• Planning and building regulations — Most extensions require building regulations approval, and some need full planning permission. We handle this for our clients, but it's worth factoring in both the fees and the timeline.
• VAT — Don't forget that VAT at 20% applies to most new build and extension work. A good builder will always quote clearly whether VAT is included or excluded.
  

Getting the Budget Right From the Start.

One of the biggest mistakes homeowners make is under-budgeting and then finding themselves making compromises halfway through the build. Our advice? Be honest with your builder from the very first conversation about what you want and what you've got to spend. A good building firm, whether it's us or anyone else, will tell you what's achievable within your budget rather than just taking the job and cutting corners later.

We'd also always recommend setting aside a contingency of around 10–15% for the unexpected. In property refurbishment and extension work, there's almost always something whether it's older wiring that needs updating or a drain that's not quite where the drawings suggested.

Ready to Get a Proper Quote?

At HD Developments, we work with homeowners across Bournemouth, Poole, Christchurch, and the New Forest to deliver home extensions that are built properly, finished to a high standard, and priced transparently.

If you're thinking about extending this year, now is a great time to get the ball rolling as the summer building season fills up fast. Get in touch today for a free, no-obligation consultation and quote. We're always happy to have a straightforward chat about what's possible.