Adding an extension is not simply a case of tacking a new room (or rooms) on to your home. You will need to think carefully about how old and new work together (both externally and internally) as well as all the cost, planning permission and project management implications.

These top tips will help you get your project off to the best start.


  • Define your objectives and prepare a realistic project schedule.
  • Plan ahead and set up a contingency fund.
  • Approach the planners first and find out what is likely to be acceptable.
  • Look carefully at finance. How much value will your extension add? What are the best deals on loans?
  • Put structural works before cosmetic and aesthetic improvements and work from the outside of the house inwards.
  • Make sure you know the difference between a quotation – a firm price and what you can expect to pay for the specified works – and an estimate, which is much less specific.
  • Always try to go for a fixed price contract.
  • Make sure the designer of your extension knows all about and allows for movement joints. With the generally dryer climate we have been experiencing in recent years buildings have been tending to move more.
  • Pay attention to the style of the windows.
  • Aim to get the shell weathertight as early as possible.
  • Check that the builder has relevant past experience and a good trading history; that he has an office address and his own headed notepaper; that he is able to offer references; and that he has third party insurance.
  • Plan your kitchen layout before you extend. It can be expensive, time-consuming and frustrating to build a new kitchen extension, only to create extra work because your dream kitchen needs waste taps or an extractor fan in a position that can’t accommodate them. Consider these practical aspects as early as you can in the process to avoid this headache.
  • Check party wall agreements — when building up to your boundary, you must notify your neighbour of your intent to build a new wall on the line of junction (boundary) and have their written permission.
  • Consider your privacy — the glazing choice may have an impact on how private your home is and how exposed you are to glances in from passersby and your neighbours.
  • If your extension will be built over or in the area of a sewer, you will need to contact your water board before work begins. You may need a ‘build over agreement’.
  • Consider an off-site construction method. Off-site construction systems, including cross laminated timber (CLT), oak frame, structural insulated panels (SIPs) and timber frame, often work well on extension projects.
  • Have a plan for where you are going to live while the work is carried out.
  • Understand the effect that removing trees or roots will have on your ground conditions.

external shot timber clad extension to traditional brick home

Paul and Fiona Abadjian added an oak-frame extension (from Border Oak), almost doubling the size of their house, and renovated the property to an award-winning standard


  • Change your mind if all all possible. It may result in a hefty budget overspend.
  • Ignore site and home insurance. Is your contractor’s cover sufficient or do you need to take out extra provision?
  • Forget the contract. It is essential to cover yourself. An exchange of letters is not sufficient.
  • Pay for any work in advance.
  • Necessarily go for the lowest quote. Busy builders may not be as competitive. References are far more important.
  • Add too many bedrooms and not enough bathrooms. A good rule of thumb is one bathroom for every two bedrooms, with an en-suite for every guest bedroom. Otherwise, end value may be affected.
  • Run into money problems. Make sure your extension is properly planned, designed and costed. If your builder has financial problems, be sure your contract is one that contains an adjudication clause.
  • Forget about building control. Before any works begin, you will need to submit either a building notice or a full plans application to building control.
  • Assume your heating will be suitable for the new, larger house. Adding an extension to your home will also add demand to your current hot water system — which may not be able to cope.
contemporary clt extension vertical wood cladding

This Adam Knibb designed extension to a listed cottage won the 2017 Daily Telegraph Homebuilding and Renovating Award for Best Extension/ Remodel

Don’t Forget to Think Ahead

With sufficient forethought you will have planned your extension so that it will both be a great source of satisfaction to everyone living in the house, and also add at least as much to the value of the property as has been spent on it hopefully considerably more.

Finally, remember that although this might be your first extension, it may not be your last. Perhaps you have enjoyed the project so much that you cant wait to undertake another!

With this in mind it will pay to put a great deal of thought into the internal design of your extension and try to avoid poky corridors, corners and awkward links that are essentially wasted space. All these will appear a great deal worse if at a later stage you choose to add a second extension. Seamless flow and clarity are the watchwords in the design of any good extension.

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