How to plan an extension so your home grows with you

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Building equipment on table

Life changes all the time but if your home doesn’t change with you, you could soon find yourself running out of space.

The thought of moving house and uprooting your life can be daunting; especially if you’ve got kids settled in the local school, it’s a good commute or you’ve got friends and family close by. What’s more, it can be a costly process - according to comparemymove the cost of stamp duty fees, estate agency fees, removal vans etc. can add up to more than £8,000, so it’s not surprising that many people may want to build an extension than go to the hassle of moving.

The added bonus is that extending your home could add long-term value to your house too.

How to extend your home

With a little imagination, there’s a multitude of things you can do to add square footage to your home, or re-organise the space you have more effectively. You could convert your loft, basement or garage into a living space, or build out with an extension.

More space can potentially give you:

  • Another bedroom, bathroom or a play area for a growing family.
  • Some independence for older kids.
  • Open plan living for a more social space.
  • An annexe for older relatives to move in.
  • A home office.

Before deciding how to improve your home, it’s a good idea to speak to an estate agent to find out what value it could add. Your home will have a ceiling price which means there’s a limit to how much value you can add, however much you increase the space. If you’re planning on selling in the short to medium term, you may want to take this in to consideration so you’re not spending more than you’ll get back.

Do I need planning permission?

You may be surprised that it’s not always necessary. In fact, according to the planning portal, permitted development rights allow you to build:
  • A single-storey side extension up to half the width of the existing dwelling.
  • A single-storey rear extension up to 4m in length for a detached dwelling and 3m long for a semi or a terrace house.
  • In certain circumstances, 3m two-storey rear extensions.
  • A loft conversion up to 50m3 in a detached house, or 40m3 in a non-detached home.

However, you can’t build extensions on more than 50% of the land around your home. This includes existing extensions and outbuildings such as sheds and garages.

Always check with your local authority or get confirmation from a qualified surveyor before you start any work.

You can find out what you need planning permission for on the Planning Portal.

If you want something on a bigger scale and find that you do need planning permission, you’ll need drawings of your plans. These often require an architect, who can help you visualise, design and implement your new space, and oversee the build.

You can apply for planning permission online from your local planning authority. They will write to your nearest neighbours and anyone else affected to notify them of your plans and allow them to comment. According to the planning portal, you’ll usually receive a decision within 8 weeks.

Do I need building regulations approval?

When extending your home, it’s likely you’ll need buildings regulation approval from your local authority. The regulations ensure a minimum standard for design, construction and alterations to buildings, to protect people’s safety, health and welfare. Often your builder or contractor will sort this out for you, but ultimately, it’s your responsibility. So speak to your builder to agree who will submit the application and ensure the inspections are carried out. You’ll receive a certificate of completion at the end of the job to confirm the work complies with regulation. The cost varies depending on the job and the area you’re in. Contact your local authority for a quote. To find out more, go to the Planning Portal section on building control.

How much does an extension cost?

The cost of an extension will vary widely depending on your location and the type, size and specification of extension you go for.

According to CheckaTrade[i] the costs below will give you a guideline for the building work. For a more accurate idea of how much an extension costs for your circumstances, get at least 3 quotes from local builders.

Building costs

Extension quality Size Cost + VAT (Range low - high) Average cost
Basic extension cost 20m2 £25k - £50k £37.5k
Standard cost for small extension 20m2 £25k - £50k £37.5k
Standard cost for medium extension 30m2 £37.5k - £67.5k £52.5k
Standard cost for large extension 50m2 £62.5k - £112.5k £87.5k
Premium extension cost 30m2 £52.5k - £100k £76.5k

Other costs to consider[ii]

  • Architect fees – around 7% of total construction costs.
  • Structural engineer fees - £500 - £1,250.
  • Surveyor fees - £700 - £1,800.
  • Planning fees - £206.
  • Party wall agreement - £1,000 - £2,000.
  • Lawful development certificate- £103.
  • Interior fittings and finishes.
  • Insurance.

How to save money on an extension

Plan plan plan! – Take the time to be sure about what you want, in as much detail as possible. Check your measurements and make detailed drawings. It will help the build to go smoothly and allow you to budget more accurately. If, at a later date, you decide that you want a new feature or to move wiring to a different spot, it will cost you time and money to re-do the work.

Keep it simple – Simple designs with straight lines will be easier and cheaper to build. Curves and unusual features will cost you more and may take more time. The same goes for your fittings, buying off-the-shelf rather than made to measure will help to keep the cost down.

Choose the right builder and agree a fixed price – Ask for recommendations from people you know and check review websites such as and Ask to visit previous clients of theirs to see the quality of work and get more information about what they’re like. Agree a fixed price upfront so costs don’t get out of control and pay in instalments.

Reuse, repair, recycle – Hold on to as much of your existing materials as you can - from bricks and floor boards to kitchen cabinets and appliances - to save buying new. If you can’t use it yourself, there might be items you can sell which will help towards your budget. And when you do need to buy something, think second hand first.

DIY if you can – If you’re confident and know what you’re doing, it can save you money to take on some tasks yourself, like project management, basic plumbing, fitting flooring and decorating. However, fixing mistakes can be costly so only take on what you can manage.

Shop around – High street suppliers can be expensive so shop around to find the best deal. Speak to your builders too as you may be able to use their trade account for some items.

Paying for the work

To pay for the work, you could use savings, borrowing or a combination of both. For larger amounts, you may be able to borrow more on your mortgage (known as an additional loan). If you’re an existing YBS mortgage holder see our additional loans - please note early repayment charges apply and other fees and charges may apply.


[i] Source: Checkatrade - 2022 How much does a house extension cost in 2022?


[ii] Source: Homebuilding - Jan 2022 How much does an Extension cost? What you'll pay in 2022