This dated 1930s bungalow was extended with a single-storey side extension and two-storey rear addition

Extending is a great way to create much needed extra space in your home, whether to accommodate a growing family or to adapt to modern living requirements, and is often cheaper than moving house.

If you’re considering extending for the first time, or are keen for project success, these 10 key rules will set you on the right path.

1. Only Extend If It Makes Sense

In some circumstances, it is better to move than to extend. If an extension will spoil the property, or will be a case of over-development (thus making an extension nonsensical from an economic perspective), then it is best to move.

But if an extension will improve the existing home, adding both space and value to the property, then it makes complete sense.

2. Make a Considered Brief

Start by defining what the new space should achieve and what problems the extension should solve. Unless you can tell your architect what is missing, then they won’t be able to make the extension more than a simple addition of rooms. This description should be more than ‘add a bedroom and bathroom’ or ‘make the ground floor bigger’.

These statements are not wrong but it helps to think more about the added benefits, for example if you are extending a kitchen:

  • Where does the light come in?
  • Can I eat my breakfast with the morning sun?
  • Do I want to sit and enjoy a view or see my garden?

The architect will solve and come up with ideas, but the best solutions come from being set the best questions from a client with challenging thoughts.

3. Choose Materials Carefully

Sometimes the materials choices are obvious and the existing house demands the extension should follow suit with a subservient extension.

If the house has no overriding character or style, then a contrast can improve both parts. The skill and challenge of the architect is to decide, with you as their client, what will work best. If you are going to contrast an existing house with materials that vary, then the solution has to be of high quality and well thought through.

A bland extension on an ordinary house is simply bland; if it also stands out because the materials look wrong, then the whole effect is ruined.

The materials are an intrinsic part of the architectural style so don’t pick materials at the end as an afterthought, or because they were ‘on offer’. Remember, you have to live with this for many years.

glass basement extension to a Grade II-Listed mews home

Artist Ruth Macdonald, was working in a cramped rented studio, so part of the brief to their architect was to add a home studio with living space above to their Grade II*-Listed home in London. The contemporary style using glass for the floor and ceiling may contrast with the traditional mews property, but allows plenty of light into the basement below

4. Get the Size Right

Getting the right size for an extension is probably the biggest challenge to the owner and the designer. There is no set rule, but many times I have seen extensions added to buildings that have spoilt the original house because they are too big and dominant.

When thinking about additional spaces, consider how they can combine or improve existing spaces, rather than simply adding more.

5. Give Thought to Architectural Style

Every house has a style, built in a certain period or with particular materials. So the architectural style that the extension takes on is important to the combined result. A Georgian house can have an extension that matches or contrasts.

If you are going to match a building, then the proportions, details and materials must be very good to make it work and appear authentic.

So is a contrast easier? Not necessarily so. If you want a modern extension on an older property, the same rules of proportion apply but the new extension must add something. This could be more glass in contrast to a solid form, or a flash of colour in contrast to a neutral palette.

6. Respect Neighbouring Properties

One of the biggest questions on a semi or a terrace property is the effect that the extension or alteration has on its neighbour’s property. Try to design your new extension with respect and awareness to neighbouring houses.

Can you avoid overlooking or overcrowding? Many extensions become so large that they appear to join up separate houses, creating terraces where they were never intended.

7. Quality vs. Quantity

So, how much will it cost to build? It is always better to build with better materials and better design than build more space of less quality. If you are going to build large and can’t afford everything, then plan to fit out the space later rather than cheapen everything.

8. Extend or Replace

Many people now alter a house so significantly that the question is not “Should we extend?” but “Should we knock it down and replace?” There is a point when you are extending when it might be easier. However, I find that reusing existing buildings, where possible, is more environmentally friendly and can provide a great base to create the dream home.

Bear in mind, though, that an extension attracts VAT at 20 per cent whereas a new build is 0 per cent, so do the sums on the costs at the beginning when you can make the right choice.

open plan kitchen extension


Penny and Douglas Dawson decided to make their open plan kitchen, dining and living space somewhere they would want to spend most of their time as a family. Their heating bills have dramatically reduced as a result of creating a glazed kitchen extension where the whole family congregates in, so they don’t need to heat the rest of the house

9. Prepare to Live Through the Mess

If you are extending and living in the house then prepare for disruption, mess and dust. Many clients have said they will live in the house and come to regret it. If you can move out and leave the builders to get on, there are real advantages:

  • The build can be faster and a shorter programme also saves money
  • The builders can turn power, water and heat off
  • They don’t have to tidy up all the time, and there are fewer arguments over the toilet

Most people have to stay while building goes on, so agree ground rules on access and use of the toilet. Seal up as many rooms as possible and invest in dustsheets to protect furniture, as plaster dust seems to get everywhere.

Above all, keep smiling throughout the process and think about why you started.

10. Assess the Difference Between Cost and Value

When you are planning your build, work through the costs at the beginning. Only you can decide what constitutes good value. The cost can be determined by the quantity surveyor or the builder but only you, the client, can say if that is worth it in monetary, enjoyment or usefulness terms.

Extension Cost Calculator

Use our free extension cost calculator tool to find out how much your extension could cost.

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