House Extension Cost

In the era of “don’t move, improve”, Scenario finds itself among the top residential architecture studios in London for high-end home extensions. We know that the House Extension Costs in London can be limiting, so we do our best to ensure every penny you spend translates to a high-functioning, high-spec package. Our distinctive and spacious house extensions – from simple single-storey extensions to larger side-return extensions, rear extensions and basement extensions – give new purpose to period properties and raise the bar for what a small architecture practice can achieve.

We established Scenario in 2007 with a vision to maximise living spaces to their greatest potential on a practical, reasonable budget. Our boutique practice takes a considered, efficient approach to our clients’ needs and desires, making the cost of building an extension accessible.

Our landmark project Scenario House, an East London Victorian home with a unique, light-filled side-return extension, was covered widely by the media and shortlisted for several prestigious architecture prizes – including the 2018 RIBA London Awards. It demonstrates our innovative, economical approach to opening up period properties for 21st century lifestyles.

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House Extension Cost

Edwardian house extension in London

House Extension Cost

Glazed house extension in London

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Starting your house extension

If you plan to tackle a house extension in London, we’re happy to help, whether you’re planning a single-storey extension, side-return extension, basement extension or rear extension.

To help save on the House Extension Cost and understand each stage of the expansion process, we’ve created a brief illustrated overview.

Most house extension projects in London require planning permission from the local council. By securing planning permission quickly and successfully, we can help keep down your House Extension Cost.


Discover if your chosen architect has had success steering planning applications through your local council. Visit the “planning applications” section of your council’s website and enter the architect’s name in the search criteria.

House Extension Cost


To help you save on your house-extension cost and understand what it takes to obtain planning permission in London, we’ve created this helpful handbook. Download it here.
Planning guide

House Extension Cost Scenario Architecture advice on House Extension Costs

Advice on the House Extension Cost, and working with Scenario Architecture

Scenario is a boutique architecture studio focused on expanding and improving homes across London and beyond. Through our bespoke architecture service, we tackle projects of every scale and complexity, from modest single-storey extensions to double-height extensions, side-return extensions, basement extensions and rear house extensions.

The House Extension Cost varies. This will depend on the scale, design, exterior materials and interior finishes of your extension. Before designing an initial concept for our clients, we discuss their big-picture aspirations and daily requirements.

When we budget the House Extension Cost on any home, we must consider the different types of extensions. Extending upward with a loft extension, for example, is often significantly cheaper than a side-return extension and rear house extension and most definitely a basement extension.

We understand our clients’ desire to understand the cost of building an extension before committing to the process. For a single-storey extension – be it a side-return extension or rear house extension – we allow between £2,000 and £2,500 per square metre, depending on finishes. These figures can also apply to renovating the existing areas within a house.

When it comes to loft extensions, it’s best to stay away from square-metre calculations as these may be confusing and misleading due to varying head-height requirements, kitchen and bathroom requirements and roof repairs. Generally we allow between £35,000 and £55,000 for loft extensions, depending on finishes.

Excavating a new basement is a great way to add significant floor space to your house with limited planning regulations. However the House Extension Cost per square metre is, by far, the highest in a basement. We allow between £3,000 and £3,500 per square metre, subject to finishes.

Early adopters of the latest drafting and building technologies, we use innovative 3D design and visualisation tools to bring down house-extension costs. Our experience has shown that real-time visualisations and virtual reality are extremely efficient tools for choosing finishes, fixtures and fittings, and simulating natural light.

We use 3D building information modelling (BIM) technology to produce reliable construction information, so all our project data comes from an accurate, coordinated 3D model. Using this cutting-edge technology collaboratively with consultants and contractors is proven to save on home-extension costs.

A Chartered Institute of Building case study featuring a Scenario project demonstrates the positive effects of this advanced technology on home-extension costs.

Scenario Architecture is a RIBA Chartered Practice. We comply with the strict criteria of the Royal Institute of Chartered Architects, covering insurance, health and safety and quality-management systems.

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House Extension Cost

Rear house extension, London

House Extension Cost

Conservation Area House Extension, London

House Extension Cost

Victorian Villa Glass Conservatory, London

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House Extension Cost In our clients words


We wanted to renovate our house in a conservation area in central London. Given this involved a complete demolition and new build with an extra floor on top, getting planning approval was always going to be tricky. Scenario did an amazing job on the new house ‘envelope’ and throughout the planning phase. We couldn’t have wished for better from them and having succeeded in gaining planning approval owe them a very big ‘thank you’.


Scenario designed a kitchen extension for us, and we are delighted with the results. The process was well organised, planned and delivered, and the whole team were a pleasure to work with. I would highly recommend them for their design, planning and project management skills.


Worked with these guys on the internal development of our office. From start to finish they were great. They got what we wanted first time and really made working with them very easy. Their aftercare has been great too helping us deal with something that they were not involved with in the first place. Will highly recommend them to anyone.

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You’ve bought a home. In and around London, that’s no mean feat. Nearly any property you purchase in the southeast will retain or accrue value. When and if it turns out you need more living space, you’ll want any enhancements you make to the size of your home to enhance your asset, too. A good architect will factor in your home’s location, layout, structural integrity and the size of your family to make your project work for you and your budget. A house extension cost shouldn’t cancel out the benefits of a house extension.

Unless your investment is a purpose-built flat, you should be able to gain extra space that suits your lifestyle. If you own just part of a house, you can built up, down or out, depending on your specific location. If you own the entire house, chances are better you can add substantial square footage. In some neighbourhoods around London, loft extensions and side-return extensions (recouping the side-rear void often left open between terraced houses) are grouped under “permitted development”, which means they don’t require planning permission from the local council if the final plans heed the extended volume requirements of the area. Avoiding time-consuming planning applications will save you money in the short run.

On the topic of money: it’s always difficult to project a house extension cost without evaluating the home in question, the resident family and their plans. But it’s likely you can nearly double the size of your home’s original square-metre floor area at a fraction of the cost of the original house, even at upwards of £2,000 per square metre. Plus you will, of course, gain a nearly new living space without paying the stamp duty and agents’ fees associated with moving house – although you may need to purchase renovations insurance.

As with most building-related activity, it is wise to source three to five quotes for a house extension. With a range of budgets to hand, you’ll be able to assess which seem low, high and just about right, and choose accordingly. The foundation, walls and roof will soak up about 15% of your costs, followed by Velux and rear glazing. You’ll want to price fixtures, fittings, appliances and any upgraded materials separately, so as not to be shocked when you receive that last bill – new kitchens are particularly dear, so keep your eyes out for deals and floor sales in the months leading up to a project. Once you’ve agreed on a budget, set up a payment plan with your architect and builders that works for everyone.