How Much Does a House Extension Cost in the UK? (2025 Guide)

Are you considering a house extension to enhance your living space? A well-designed extension can increase your home’s functionality, improve energy efficiency, and boost its value. However, understanding the costs involved is essential for planning effectively.

As an architectural designer with clients across the UK, I help homeowners bring their extension ideas to life. In this guide, I’ll break down 2025 cost estimates, key pricing factors, and how to budget smartly.

1. Key Factors Influencing House Extension Costs

Several factors determine the total cost of the extension to your home:

• Type of extension: Single-storey extensions are generally more affordable than double-storey designs, which require additional structural support

• Size & scope: Larger extensions naturally cost more, and complex designs may increase construction expenses

• Location: Building costs are higher in London and the South East due to demand. Conversely, they are more affordable in northern and rural areas.

• Materials & finishes: Opting for bespoke glazing, high-end materials, or premium fixtures will increase your budget

• Planning & regulations: Some extensions require planning permission and building regulation approval, adding to the overall cost

At FORMA Architecture, we help you navigate these cost factors with tailored design solutions that balance your vision, budget, and planning requirements. Our expertise ensures a smooth process from concept to completion, helping you maximise value while avoiding costly surprises. To learn more, please feel free to contact us at hello@formaarchitecture.co.uk

2. Average Cost of a House Extension in 2025

Understanding the costs involved is crucial when planning a house extension. Below is a breakdown of estimated costs to help you budget effectively and make informed decisions about your project:

• Single-storey extension: Approximately £1,800 - £2,800 per m² = £27,000 - £50,000 total

• Double-storey extension: Approximately £2,000 - £3,200 per m² = £50,000 - £85,000 total

• Wrap-around extension: Approximately £2,200 - £3,800 per m² = £60,000 - £120,000 total

💡 London and South East costs tend to be 15-25% higher than the national average.

💡 Loft conversion costs typically range from £25,000 to £60,000, depending on the size and complexity.

📌 Note: These figures are indicative estimates based on industry sources such as Checkatrade, RICS, and recent contractor data. Actual costs can fluctuate due to local labour rates, material prices, and project complexity. Always obtain multiple quotes for a more accurate budget tailored to your specific location and requirements.

3. Planning Permission & Building Regulations

Do I need planning permission?

Many extensions don’t require planning permission under Permitted Development (PD) rights. They must however, meet specific size and design criteria:

• Single-storey rear extensions can extend up to 3m for terraced and semi-detached homes without permission, and 4m for detached homes. Larger extensions up to 6m (terraced/semi) or 8m (detached) may be allowed under the Neighbour Consultation Scheme

• Double-storey rear extensions are not permitted under PD and require full planning approval.

• Side extensions can be up to half the width of the original house but must be single-storey and no taller than 4m

• Wrap-around extensions almost always require planning permission due to their impact on the building’s footprint

• Height restrictions: Extensions cannot be taller than the existing house. If they are within 2m of a boundary, they can also not exceed 3m in height

📌 Please note that permitted development rights can be subject to change, and local planning authorities may have additional restrictions, especially in designated areas like conservation zones.

📌 Flats, maisonettes, and homes in conservation areas or listed buildings do not benefit from PD rights, meaning full planning permission is required for extensions or alterations. It's always advisable to check the latest regulations or seek professional advice before proceeding.

What about building regulations?

Regardless of planning permission, all extensions must comply with UK Building Regulations, covering:

• Structural integrity (foundations, beams, load-bearing walls)

• Insulation & energy efficiency

• Fire safety & ventilation

• Drainage & electrics

📌 Expect to pay between £300 - £1,000 for approval and inspections.

At FORMA Architecture, we help you navigate these requirements, ensuring your extension complies with regulations while bringing your vision to life. To get in touch with us, visit our contact page, or email us at hello@formaarchitecture.co.uk

4. Architectural & Design Fees

Hiring an architectural designer ensures your extension is both functional and aesthetically pleasing. Typical costs include:

• Architectural design fees: £50 - £100 per hour or a fixed fee based on project scope

• Structural engineer fees: £500 - £2,000 (for major load-bearing work)

• Planning application fees: £258 in England (higher for larger projects)

📌 Investing in professional design upfront can save you thousands by avoiding planning rejections, costly mistakes, or poor space planning.

Bring your home vision to life

Ready to create a home that reflects your lifestyle, values, and needs? Let's make it happen together. Whether you want to build, renovate, or extend, we’ll guide you from initial concept to planning approval and construction. Schedule a free 30-minute consultation to get started.

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5. Budgeting for your House Extension

Once you have a cost estimate, consider how your extension fits within your overall budget. Many homeowners fund their projects through:

• Personal savings: The simplest option with no interest or repayment concerns

• Remortgaging: Unlock equity in your home to fund major renovations

• Home improvement loans: A flexible option with fixed repayments

💡 Need guidance on project costs? Book a free consultation, and I’ll help you plan your extension within your budget!

6. Our Approach to Designing your Extension

Every home is unique, and a well-designed extension should feel like a seamless and natural addition, rather than an afterthought. Our approach prioritises:

• Maximising natural light: Using glazing, skylights, and open-plan layouts to create bright, welcoming spaces

• Efficient layouts: Ensuring the new space flows well with the existing home, avoiding awkward transitions

• Material choices: Blending new and old elements for a cohesive design that complements your home’s character

• Cost-conscious solutions: Balancing design ambition with budget considerations, helping you get the best value with cost-effective design choices

Whether you’re looking for a kitchen extension, loft conversion, or full-scale renovation, we will help tailor your project to your needs, budget, and long-term goals.

Create a Home that Works for you with FORMA Architecture

A well-planned house extension is more than just extra space – an extension enhances your home's liveability, and adds value to your home. Whether you’re expanding for a growing family, designing your dream kitchen, or adding value before selling, we can help bring your vision to life.

As an architectural designer working remotely across the UK, I offer expert guidance from concept to completion. Let’s chat about your ideas and create a plan that works for your home and budget.

Let’s Create Something Exceptional

A well-designed home is an investment in your lifestyle and future. With expert planning, thoughtful design, and efficient execution, we ensure your project meets your vision while staying cost-effective. If you're ready to take the next step, let's discuss your ideas and turn them into reality.

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