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House extensions: 15 things everyone needs to know

10 min read
Oct '25 • by Heather

Quick summary

House extensions typically cost between £1,000 and £3,000 per square metre, depending on quality and size, with a full build taking around 7 to 15 months. Remember, costs can include hidden fees like planning, surveys, and legal work, and the timeline can vary if planning permission or party wall agreements are needed.

Before you embark on extending your home, there are a few things every homeowner should sit down and consider. After all, even modest additions are big projects to undertake and the last thing you want to encounter is surprises.

If you’re wondering where to start with your project, here’s our breakdown of everything a homeowner needs to consider before extending their home…

Will it add value to your home?

Even if you’re not planning to sell your home anytime soon, it’s important to understand what your project may or may not do for the value of your home. Extending is an expensive project but can, in many cases, reward your investment when it comes to putting your home back on the market.
How much value you add will depend on…

  • The quality of the work
  • What (if any) additional rooms you’ve created
  • Local ceiling prices

On this last point, you will find that some areas suffer from low ceiling prices, which means there’s a limit to how much value you can add.
To work out if your home is affected by a ceiling price, research the prices in your area and see what patterns emerge.

Try our free project value calculator.

Have you got enough budget?

You’ve put money aside, but have you got enough for everything required? While many people plan for the contractor fees and materials, there are a lot of hidden costs that some aren’t prepared for. Such as…

  • Legal fees
  • Stamp duty and land tax
  • Measured surveys
  • Architecture fees
  • Structural engineer fees
  • Planning application fees
  • Building regulation fees
  • Warranty
  • Services
  • Waste removal
  • External works

However, don’t let this put you off. Architects can often advise on pricing and there are brokers who specialise in home development loans, should you want to utilise the equity in your property, rather than rely on savings.

Learn more: How much does an extension cost?

How much does an architect charge?