Home Extension in London: Your 2025 Planning Guide

Rear extensions are ideal for homes with ample garden space. They extend from the back of the property. This makes them great for kitchen-diners or open-plan living spaces that link to the outdoors. Often, they are covered by Permitted Development Rights. However, this depends on factors like size, materials, and your local authority's planning policies.

If you own a Victorian or Edwardian terrace with a narrow side alley, you can add a side return extension. This change will turn that wasted space into a bright and functional part of your home. These extensions often go hand in hand with internal renovations to help create wider and more open interiors. We guide clients through this as part of our fully managed design and build process.

Wraparound extensions combine rear and side return layouts, giving a notable increase in ground floor space. This option is especially favoured by homeowners wanting to create large, versatile family spaces. However, it often requires full planning permission and structural reconfiguration.

Planning to stay long-term or increase your home's value significantly? A double-storey extension adds space on both ground and first floors - perfect for extra bedrooms, bathrooms, or a home office. These projects need more planning, but they offer the best return on investment when done well.

Loft conversions are a cost-effective way to add space in London, especially when you can't expand outdoors. From dormer to mansard styles, lofts can be converted into bedrooms, studies, or even small apartments. As bespoke extension specialists, we ensure your new space blends seamlessly with the rest of your home.

In busy urban areas with limited outdoor space, basement extensions are a great option. They are ideal for creating gyms, media rooms, or guest suites right below your home. However, they are structurally complex and typically the most expensive option. In addition, they also require longer timelines and specialist planning permissions.

Many single-storey extensions can be built under Permitted Development Rights. This means homeowners don't need formal planning permission. However, this only counts if your design meets specific criteria. These criteria include limits on height, volume, materials, and distance from boundaries.

Interested in larger or more complex projects, like a wraparound or double-storey extension? If so, you'll almost certainly need to apply for planning permission. And if your property is listed or located in a conservation area, expect tighter controls on materials and structural changes. You can check the latest criteria via the Planning Portal or allow us to manage the entire process as part of your design package.

Regardless of planning requirements, Building Regulations approval is always mandatory. This ensures structural safety, fire resistance, energy efficiency, and ventilation meet legal standards. At SM London Construction, we collaborate with your local authority and approved inspectors. This helps ensure that every build is compliant and free of stress.

London extension costs vary widely depending on size, complexity, location, and specification. Based on recent 2024--2025 industry data from Checkatrade, here are average starting points for common extension types:

• Rear or side return extension: £45,000 - £80,000+
• Loft conversion: £35,000 - £65,000+
• Wraparound or double-storey: £70,000 - £120,000+
• Basement extension: £150,000+

Finishes, glazing, planning consultancy, and structural reinforcements can add to the overall cost. During your consultation, we'll provide a fully itemised, fixed-price quote to match your vision and budget. You can also view similar projects and breakdowns on our news page.

The average construction timeline for a London home extension ranges between 8 to 16 weeks, depending on the scale of the project. Before building starts, you'll also need to allow time for design, approvals, and preparation--typically another 4 to 8 weeks.

Here's a General Timeline Structure:

1. Consultation & Survey
2. Planning Approval (if needed)
3. Final Designs & Building Regulations
4. Construction Phase
5. Finishing & Snagging

We aim to minimise disruption throughout the process. In many cases--especially with single-storey or loft builds--you can remain at home while work takes place. We'll advise on this during your consultation and build around your schedule whenever possible.

With so many possible layouts and design options, choosing the right home extension in London can feel daunting. That's why we help clients focus on three core decision areas: purpose, return on investment, and the right team to bring their ideas to life.

What Do You Want to Achieve?

Start by thinking beyond square footage. Are you creating space for a growing family? Planning a luxury kitchen or entertaining area? Adding a home office or guest suite? Each scenario points to a different type of extension.

For instance, a rear kitchen extension suits open-plan living and social zones. A loft conversion is ideal for peaceful, private workspaces or bedrooms. If you're looking to boost rental income or resale value, a double-storey extension may be worth the higher initial investment. The key is aligning your build with how you plan to use your home now--and in five years' time.

We'll explore all this with you during your consultation to make sure every recommendation supports your future goals.

Should You Consider Return on Investment?

While most people extend for practical reasons, the long-term financial return is worth considering. Rightmove's 2024 property data reveals that a well-planned extension in London can boost a home's value by 10% to 20%, depending on location and the kind of upgrade.

Extensions that add bedrooms or improve flow between living spaces deliver the strongest returns. That said, quality matters. Premium finishes, smart layouts, and smooth integration with your home's style boost both your lifestyle and its value.

Who You Choose to Build It Matters

At SM London Construction, we provide a complete end-to-end service from concept to completion making the process with us seamless for busy professionals and careful homeowners. You'll work with the same dedicated team for your project. They'll support you from the first site visit to planning, design, construction, and final touches.

Our teamwork ensures your vision comes to life. We manage every detail, from planning permission to paintwork, with care. Learn more about how we deliver complex, high-spec projects through our proven design-and-build process.