How To Plan A House Extension

Whether you’re hoping to expand your home, create a new room, or improve functionality, a house extension can be an exciting and transformative project. In this guide, we’ll walk you through the essential steps of how to plan a house extension. We’ll also discuss considerations to ensure a successful and well-planned extension, that seamlessly integrates with your existing home. Let’s dive in!

How to plan a house extension


Assess your requirements 

Before embarking on a house extension, it’s crucial to determine your specific needs and requirements for the space. Are you looking to add a new bedroom, create a home office, or expand your living area? Understanding your goals will help you make informed decisions throughout the process.

Understand your local regulations

Familiarise yourself with local building regulations and planning permissions in your area. Consult with your local planning authority or seek professional guidance to ensure compliance with regulations and avoid any potential setbacks. It’s important to understand the limitations and requirements specific to your location before proceeding with your extension plans.

Planning requirements are dependent on the design itself, location and property type. Most properties have permitted development allowances that make extending, in theory, more straight forward. Factors like materials, scale, neighbouring context, overlooking, the location (conservation area, greenbelt) will all affect the application type. The planning portal has content and all councils make searching planning applications accessible so you can see locally what permission others have applied for.

Budget Planning

Establishing a realistic budget is essential. Consider the costs associated with design, construction, materials, labour, permits, and any additional expenses that may arise. Research current market rates and consult with professionals to get accurate estimates. It’s advisable to set aside a contingency fund for unexpected costs or changes during the project.

What costs do I need to factor into my budget? 

While a large part of your budget will go towards construction and the materials involved in building your rear extension, there are other costs to consider. For example, a typical budget is often split like this:

  • Architects – 4% (based on Resi’s services)
  • Administration fees – 1%
  • Structural Engineers – 3%
  • Surveyors – 2%
  • Contractors – 30%
  • Materials – 20%
  • Fittings – 5%
  • Glazing – 15%
  • VAT – 20%

How much does a rear extension cost per square metre? 

Our friends over at Resi, the UK’s number one home extension company, note that based on an average rear extension size of 45m2, the average cost of a ground floor rear extension in London is between £85,500 and £135,000. For the rest of the UK, the average costs are between £74,250 and £117,000.

Taking these averages, you can work out the likely cost of your rear extension using the estimates below.

  • £1900 = Average lowest cost per m2 of rear extensions in London and the South East
  • £3000 = Average highest cost per m2 of rear extensions in London and the South East
  • £1650 = Average lowest cost per m2 of rear extensions for the rest of the UK
  • £2600 = Average highest cost per m2 of rear extensions for the rest of the UK
  • For example, if you’re planning on a small rear extension of 30m2, your build will likely cost between £57,000 and £90,000 in the London area.So, why is there such a difference between the lowest and highest amounts? The final price depends entirely on the complexity of the build, such as the type of roof and finishes you want, the materials you use, and the building company you choose to construct your design.

    NB: The m2 depicted is the total refurbished area, not just the additional space. For example, a 45m2 extension will typically add an extra 15m2 of space. Consequently, the above calculations include the room the extension is being attached to.

Engage professionals 

Working with experienced professionals, such as architects and interior designers, can simplify the extension process. Collaborating with experts ensures that your vision is translated into a well-executed design and that all technical aspects are addressed properly. Research and choose professionals with a proven track record and a thorough understanding of your specific project requirements.

Sustainability & and Energy Efficiency 

Incorporating sustainable practices and energy-efficient features in your extension can benefit both the environment and your wallet. Consider eco-friendly materials, energy-efficient lighting, proper insulation, and the integration of renewable energy sources like solar panels. These choices can reduce your carbon footprint and lead to long-term energy savings. 

Construction & project management  

During the construction phase, effective project management is essential to ensure smooth progress. Choose a reputable builder who will oversee the construction, coordinate subcontractors, and ensure adherence to timelines and quality standards. Regular communication with your project manager and builder will help address any concerns promptly and keep the project on track.

Finishing Touches

As the construction nears completion, focus on the finishing touches that will bring your vision to life. Consider interior design elements such as paint colours, flooring,fixtures and furnishings. Working with an interior designer will ensure you create a cohesive and inviting space that complements your new extension. 

Enjoy your extended home 

Adding a house extension is an exciting opportunity to enhance your space and improve your home’s functionality. Remember to plan diligently, engage professionals, consider design elements carefully, and enjoy the journey of transforming your house into a dream home with a well-executed extension. Once construction is complete and all the finishing touches are in place, it’s time to enjoy your beautifully extended home!

In Sum 

If you’re looking for expert advice and guidance on your renovation, don’t hesitate to contact us. Pop us an email at contactus@topologyinteriors.com. Our experienced team of interior designers would be happy to help you create the home of your dreams within your budget. 

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