The Grand Budget: What Will It Cost?

As with any good architectural adventure, the cost of your house extension is subject to a dizzying array of variables. Location, size, complexity, materials, and that ever-unpredictable British weather will all have their say.

A basic single-storey extension in the UK will typically cost between £1,500 and £2,500 per square metre on average but a lot depends on where you live and you could find yourself paying considerably more than this. If your aspirations include high-end finishes, bespoke joinery, or a glazed wall that elegantly blurs the boundary between indoors and out, expect to pay £3,000 per square metre or more.

For those seeking to climb the extension ladder, a double-storey addition can provide much-needed space while keeping costs more efficient. Prices here range from £2,000 to £3,000 per square metre, with economies of scale making the upper level a more cost-effective proposition.

If you are extending into the loft, prices can vary wildly depending on whether the space requires full structural reinforcement or just a simple dormer. Expect to spend £50,000 to £80,000 for a typical conversion, with luxury versions reaching beyond £100,000.

And then there’s the kitchen extension – a phrase that sends shivers of delight down the spine of a typical home improver. Yet even these can be costly per square metre, particularly if they involve structural knock-throughs, significant plumbing, or bi-fold doors leading to your ‘entertainment-ready’ terrace. Here, you could be looking at £60,000 to £100,000 depending on specification.

The Hidden Gremlins of Costing

Budgets and reality rarely align perfectly in construction projects. Here are some of the common cost creepers that could send your meticulously planned budget into a tailspin:

• Planning Permissions & Building Regulations – Depending on your project, you may need planning permission, which can cost from £200 to £500. Building regulations approval, necessary for structural and safety compliance, will add another £750 to £2,000.
• Structural Engineer Fees – If you’re removing walls or adding loads to the existing structure, expect to pay between £500 and £2,000 for the expertise of a structural engineer.
• Party Wall Agreements – If your project affects a shared wall, you may need a party wall agreement, which can cost £700 to £1,500 per neighbour.
• Unexpected Groundworks – Once the diggers arrive, all bets are off. Poor soil, drainage issues, or long-forgotten Victorian sewer lines can add £5,000 to £20,000 to your build.
• VAT – Residential building work attracts 20% VAT, though certain types of conversions and self-build projects may qualify for a reduced rate.
• Project Management Costs – If you’re bringing in a project manager or architect to oversee the work, budget an additional 8–10% of total build costs.

Ways to Keep Your Budget in Check

So how do you ensure that your extension doesn’t spiral into a money pit worthy of a Channel 5 house extensions from hell documentary? Here are some golden rules:

• Plan, Plan, and Then Plan Again – The more detailed your plans, the less room there is for expensive surprises.
• Get Multiple Quotes – Always obtain at least three quotes from builders to compare pricing.
• Choose Materials Wisely – Structural glass and zinc cladding may be the stuff of design dreams, but savvy choices such as composite materials and standardised sizes can cut costs dramatically.
• Consider a Shell-Only Build – If you’re comfortable with DIY or hiring trades yourself, having your builder complete just the structural work can save thousands.
• Be Realistic About Extras – Underfloor heating, smart home tech, and bespoke cabinetry are delightful, but they all add up. Prioritise the must-haves over the nice-to-haves.

Conclusion

A house extension is a thrilling venture that, when done right, can transform not only your home but your entire way of living. Yet, as we’ve seen time and again, the line between triumphant success and financial disaster is a fine one. The key? A well-researched budget, a contingency fund of at least 10-20%, and the resilience to handle those inevitable moments of sheer terror when the builder calls with bad news.

But in the end, as you step into your sun-drenched, beautifully extended space – whether it’s a sleek kitchen diner, a lofty new bedroom, or a light-filled office with garden views – you’ll know it was worth every painstakingly calculated penny.