How Much Does a House Extension Cost in 2025?

If you’re running out of space at home, consider a house extension instead of moving. But before you start calling builders, it’s essential to ask one key question: how much does a house extension cost? The truth is, the price can vary a lot depending on several factors, including size, type, location, and design choices.

This detailed guide will walk you through everything you need to know, from cost per square metre to budget planning tips. Whether you’re looking to build a new kitchen, add a bedroom, or convert your garage, we’ve got you covered.

Why Consider a House Extension?

House extensions are a popular way to improve your home without the hassle of moving. They can help you:

• Gain extra space for growing families
• Create an open-plan kitchen or living area.
• Add value to your property.
• Avoid estate agents and legal fees

With careful planning, a house extension can be more affordable and less stressful than relocating. But how much should you budget?

What Is the Cost Per Square Metre?

Understanding the cost per square metre is the best way to start estimating your extension project. This method helps you compare different types and sizes of extensions easily.

Here are typical UK rates:

• Basic shell only: £1,200 to £1,700 per square metre
• Fully finished extension: £1,800 to £3,000 per square metre

The difference lies in what’s included. A shell extension gives you the outer structure with walls, roof, and foundation. A full extension includes internal finishes like electrics, plumbing, flooring, and plastering.

Average House Extension Costs by Type

To get a clearer picture, let’s break down the average costs based on extension types:

These figures assume average materials and a mid-range finish. Costs can rise quickly with high-end features or complex designs.

What Influences the Final Cost?

1. Size of the Extension

The larger the extension, the higher the total cost. However, bigger projects may offer better value per square metre.

2. Design Complexity

Simple layouts are cheaper. Adding curves, split levels, or special roofing styles can increase prices due to the increased labour and materials required.

3. Location

Building costs are typically higher in London and the South East compared to the North or rural areas.

4. Ground Conditions

If the soil is poor or you need to move drains or tree roots, the groundwork will cost more.

5. Access

Limited access to your site could mean extra time and effort for the construction team, which affects the price.

6. Planning and Legal Requirements

Although many extensions fall under Permitted Development, others require full planning permission. Applications and legal fees can add hundreds or even thousands to your budget.

7. Finish and Features

Luxury tiles, underfloor heating, or bespoke joinery can significantly raise the total spend.

Real Example: Kitchen Extension Budget Breakdown

Let’s look at a realistic budget for a single-storey kitchen extension of 30 square metres.

Always include a contingency of 10 to 15 %  for unforeseen expenses like delays or extra materials.

How Long Does a House Extension Take?

Project timelines vary based on the scope of work, but here are average durations:

• Small single-storey extension: 10 to 12 weeks
• Two-storey or complex extension: 16 to 20 weeks
• Conversions or basements: up to 6 months

Make sure to allow extra time for planning permission, drawing approvals, and possible weather delays.

Do You Need Planning Permission?

Not all extensions need full planning permission. Many fall under Permitted Development Rights, which allow specific changes without formal approval.

In general, you may not need permission if:

• The extension does not exceed half the size of the original house
• It’s not taller than the existing roof.
• It’s built at the back and not near protected land.

However, you must always get building regulations approval. This ensures that your extension meets structural safety, fire, and energy standards.

Tips to Reduce House Extension Costs

You can save money without sacrificing quality. Here’s how:

1. Keep the Design Simple

Stick to a square or rectangular shape. Straight walls and simple roofs are more affordable to build.

2. Reuse Where Possible

Reclaim doors, timber, or bricks from existing structures. Look for reclaimed materials at salvage yards.

3. Project Manage Yourself

By coordinating trades, ordering materials, and handling day-to-day decisions, you could save up to 20%.

4. Choose Off-Season Building

Contractors may offer lower rates during quieter months like winter or early spring.

5. Get Multiple Quotes

Always compare at least three builders. Be sure to check references and insurance before hiring.

Will a House Extension Increase My Property Value?

Yes, and in many cases, the added value exceeds the cost of building. According to property experts:

• An extra bedroom can add up to 12% to your home’s value
• A well-designed kitchen extension can increase value by 10%
• Adding a bathroom could boost value by around 6%

Buyers often pay more for homes with spacious, modern layouts. An extension can also make your home more attractive in a competitive market.

Hidden Costs to Be Aware Of

It’s easy to overlook some of these expenses:

• Planning application fees
• Party wall agreements if you share a wall with a neighbour
• Scaffolding or structural supports
• Higher insurance premiums during the build
• VAT, which may apply if you are using contractors

Make sure your builder provides a detailed quote so there are no surprises along the way.

Financing Your Extension

If savings aren’t enough to cover the cost, you may consider:

• Remortgaging to release equity
• Home improvement loans
• Credit from kitchen or bathroom suppliers

Just make sure the interest rates are affordable and compare lenders carefully.

Final Thoughts

So, how much does a house extension cost? As you can see, it depends on a wide range of factors. A simple garage conversion might cost under £10,000, while a large two-storey extension could be over £150,000.

The key is to plan carefully, get advice from professionals, and always have a cushion in your budget. With the right approach, your extension can bring years of added comfort, functionality, and value to your home.