One of the first things we’re often asked is, “What’s it going to cost?” And while the answer depends on lots of factors, from your finish level to where you live, we can offer a realistic starting point.

This article will help you understand what makes up the cost of an extension, where budgets often go off track, and how to shape your project around what you can afford.

BUILD COSTS: THE BASICS

The most useful place to start is the cost per square metre. This gives a broad idea of what your main building shell will cost before you start adding kitchens, bathrooms, or specialist finishes.

As a rough guide for 2025:

• £1,700–£2,200/m² – Cost-effective local builder, simpler finishes, minimal complexity
• £2,200–£2,600/m² – Mid-range finish with some bespoke elements
• £2,600–£3,000+/m² – High-end detailing, larger glazing, or more complex construction

This assumes you're working with a competent builder, using standard materials, and building to current regulations. It doesn’t include VAT (if your builder is VAT registered), professional fees, or any reconfiguration to the rest of your home.

WHAT ELSE ADDS TO THE COST?

Build cost is just the beginning. Here are other common elements to allow for:

• Kitchen fit-out: £15,000 – £30,000+
• Bathroom fit-out: £3,000 – £6,000+
• Glazed doors (bifold/sliding): £1,500+ per metre (installed)
• Rooflights: £500 – £2,000+ each
• Built-in joinery & finishes: Varies widely – bespoke units, panelling, lighting, flooring
• Landscaping: Often overlooked but can make a huge difference
• Professional & application fees: 10–18% of build cost (including architectural, structural, planning and Building Regs fees)

Also consider things like waste removal, temporary roofing, utility upgrades, or new drainage — they can all add to the total.

WHERE COSTS START TO CLIMB

Some of the most common budget pitfalls we see:

• Choosing layout before knowing your budget – Design decisions should be guided by what you can afford, not just what you dream of
• Changing the spec mid-project – Switching to high-end finishes late on is one of the easiest ways to blow your budget
• Not allowing a contingency – We recommend 10–15% extra for unexpected costs, especially on older properties
• Skipping early professional advice – Making the wrong assumptions about planning, structure, or materials can lead to costly redesigns
• Choosing the wrong Architect – We often pick up the pieces where clients have had false starts with other design practices

One client we worked with originally planned a 50m² extension at £1,800/m². Once they added bespoke kitchen joinery, large format glazing, and zinc roofing, the cost rose closer to £2,800/m². A smart redesign helped trim costs without compromising the space they needed.

WAYS TO STAY IN CONTROL

• Start with a realistic ballpark – It’s much easier to scale back than to stretch too thin
• Be clear on your priorities – Are you investing in space, finishes, or long-term flexibility?
• Work with your architect – We’ll help you spot what’s driving cost and where to adjust
• Use a fixed-fee design process – This keeps professional costs transparent and manageable
• Get early builder input – We often recommend a cost review at concept stage, especially for larger or more ambitious projects

SHOULD I PHASE THE WORK?

In some cases, yes — especially if your budget won’t stretch to everything in one go.

For example, we might:

• Design a kitchen extension now, with future provision for a loft conversion
• Focus on key living spaces first, leaving finishes or landscaping for later
• Build structural openings now and complete internal layouts in stages

But phasing only works well with good planning. Poorly sequenced works can end up costing more.

CLOSING THOUGHTS

Knowing what your extension might cost isn’t about locking yourself into fixed numbers — it’s about designing with your eyes open. With the right guidance and clear priorities, most budgets can be made to work.

If you’re thinking about extending and want early cost advice, get in touch for a free consultation or see our Getting Started Guide for more details on how we help clients plan smarter.