Building an Extension – What to Consider

With rapidly spiralling house prices forcing more people to stick with their current property for the time being, a number of homeowners are choosing to extend their existing property in order to gain extra space without having to sell up. Although the cost of building an extension can be significant, it is a sure-fire way to add value to your home and will serve those looking to move away in a few years time just as well as those who are setting up for the long haul.

As with any major project, there are a lot of factors that need to be carefully considered before any work on an extension can be undertaken. From initial designs to planning applications, supplies to labour costs, each stage of the process has the potential to cause major disruptions to the project if it is not thought through before hand.

Design and planning

It is important to keep in mind the 'look and feel' of your existing property when designing an extension. Try to match the most prominent features, such as the roof, with those of the existing building and use similar materials where possible to ensure a sense of continuity. It is also a good idea to make sure that any new doors and windows line up with existing fixtures.

Think about the neighbours

During the planning application process, neighbours will be able to air any misgivings regarding your proposed extension, so it is a good idea to ensure that the extension will blend in with the other properties on the street. Getting neighbours involved during the initial stages of design can save a lot of time and money later down the line if they do have any objections or concerns. Even if you are legally entitled to make changes that they oppose, it could be better to compromise than make potentially life-long enemies.

Hiring an architect

People can visit an architect for advice on what is and is not feasible with the space at their disposal, once they have an idea of what they want their new extension to entail. Qualified architects will also be able to provide extension advice, on issues concerning planning permission and any local factors that may influence the modifications that can be made to a property, as well as drawing up detailed specifications. Many may offer to oversee the building of the extension as project manager, for an additional cost.

Planning permission

For most people extending a property, a successful application for planning permission will be necessary before any work can be undertaken. Planning seeks to control the way that neighbourhoods, towns and cities are developed, focusing on the way that land is used, the appearance of buildings, landscaping considerations, road access and the impact that a development will have on the environment. You will definitely need planning permission if:

  • The proposed extension is higher than the highest part of your original roof
  • Any part of the proposed extension is over four metres high and within two metres of your property boundaries
  • The ground area covered by the proposed extension covers more than half of the total property area (excluding ground covered by the original building)
  • The total volume of original property is increased by more than 115 cubic metres (volume is calculated using the external dimensions of the structure: length x breadth x height)
  • The total volume of original property is increased by more than 10% (or 50 cubic metres, whichever is greatest) for terraced houses, or any property in a conservation area, a national park or the Broads.

The planning process

Once the council has received your plans, it will place them on the Planning Register for public viewing and notify neighbours. A committee appointed by the local council will then make a decision or appoint a senior planning officer to make one for them. The process can take up to eight weeks, and if permission is granted, planned works must be completed within five years.

If permission is denied, plans can be amended to take account of any problems raised by the council, then resubmitted within 12 months without any further charge. Appeals can be lodged within three months of the council's decision. Those who fail to apply for planning permission before building an extension may face heavy fines and be required by law to demolish any new building work.

More information regarding planning permission can be obtained via local authorities, or by visiting the Government's Planning Portal website. Many architects offer to take care of the planning application process and any appeals/amendments for an additional fee. Alternatively, homeowners can seek the services of a specialist planning consultant.

Building Regulations

In most cases, it will be necessary when extending a building to ensure that everything done complies with the Government's Building Regulations. Unless the planned extension is a porch, conservatory or detached garage built at ground level, taking up less than 30m2 of floor space, it's likely that Building Regulations will be relevant.

These apply to any building work in England and Wales, and set minimum standards for the design and construction of buildings to ensure that they comply with health and safety rules. They also include certain requirements involving fuel and power conservation, and ensure that access and facilities are provided for people with disabilities in public buildings.

A reputable builder will be able to help you in ensuring that your extension meets all the Building Regulations that may apply to the work, and in making sure that any inspections required are completed and work meets the and construction and performance standards set by the regulations.

How to comply

Building Regulations approval is a separate process from obtaining planning permission for your extension, and it is extremely important to ensure that your extension is approved in both regards. To achieve compliance with Building Regulations, you or your architect must submit full and detailed plans of the proposed extension, together with the appropriate application form and fee, to your Local Planning Authority..

Building control surveyors will examine the plans to guarantee that they accord with Building Regulations, and will approve the project. Your builders are required to notify the council when building work commences and at various stages throughout the project, and inspections can be made to ensure that work, such as laying foundations, damp proofing and installing drains is carried out to standard. A final inspection will be made upon completion of the extension, and a certificate of completion will be issued if everything is up to scratch.

Information about how Building Regulations might apply to different types of extensions can be found through the Government's Planning Portal, or via your Local Planning Authority.


If a dispute arises between a person carrying out an extension subject to Building Regulations and the local authority regarding the building work's compliance with regulations, then determinations can be used to adjudicate.

A determination is a decision made by the Secretary of State for Transport (in England) or the National Assembly for Wales to resolve the matter when a local authority says that your plans do not comply with the Building Regulations, but you believe that they do. Most disputes are settled with a determination, one way or the other, so it is important to apply for one as soon as the need arises, and before substantial construction has begun.

Dispensations and Appeals

In some cases, you will be able to apply to the local authority for a relaxation to, or dispensation from a certain regulation if you believe that it is inapplicable to your extension. Your application must be made within 28 days of notification of failure to comply with Building Regulations and, if they refuse your applications, you have the right to appeal to the Office of the Deputy Prime Minister (in England) or the Welsh Assembly Government (in Wales).

Labour and supplies

When picking your workforce, it is important that you avoid cowboy builders who may be prepared to charge you less, but will cut corners, perform shoddy work and will end up costing you more in the long-term, especially if you ever come to selling your house. Reputable tradesmen and builders can be found through the Federation of Master Builders (FMB) and will be able to provide you with advice on which Building Regulations you will need to adhere to.

Building an extension requires a lot of work, and it would be prudent, for those who can afford it, to invest in a professional project manager to oversee the entire process. If you can afford a project manager, they will take care of organising any labour and supplies that will be needed.

Finding the right builder

For those searching for a building contractor, the best way to proceed is to talk to several different contractors and ask for quotes for the extension. Make sure that all building supplies are included within the quote. While it may be the case that you want to use specialised materials in the building of your extension, discuss these with your builder as they may be able to secure discounted prices. Also ask them if you can talk to some of their customers and see some of their previous work to ensure their quality.

Once you have picked your builder, it is vital that you make sure you know the terms of your arrangement by completing a proper contract. A sample contract is available on the FMB website. You should agree on a timetable with a predefined completion date, although it is best to keep this flexible if possible, as it may be subject to the availability of certain materials, weather or changes you make to the extension's plans. You shouldn't need to pay a deposit as most builders have a cashflow and credit with local building suppliers, but if you do provide a down payment, agree to pay only for the first stage of work in advance. Once the extension is finished, make sure you are fully satisfied with the work before paying the outstanding fees.

Remember: You're the boss!

Throughout the course of the project, you may be forced to deal with a large number of labourers, including builders, plasterers, plumbers, roofers and other specialists. The most important thing is to remember is that you are in charge and it is your money. Don't be afraid to ask what someone is doing and don't be afraid to say 'no'. If you want to make changes once a project has started, make them with your project manager or the building contractor you hired, not the team carrying out the work. Specialist trades people will be able to offer you quality advice, but the decision is ultimately yours, and you should not feel pressured into doing something you don't want or can't afford to do.

Budgeting for an extension – Watch your wallet

The extension budget is one area where most people fall down, often underestimating the time and money that is required to complete the project. Adequately budgeting for an extension is one of the most important things to consider when planning a project to improve your home. Here are a few tips to remember:

  • You should be realistic in what you want to achieve.
  • Don't over-stretch yourself financially. Only take on a project that you feel you can comfortably manage.
  • Try to leave some money aside for emergency funds in case anything goes wrong or any changes need to be made.
  • While doing as much of the work yourself as possible could save you on labour costs, it will take much longer and the quality of the extension could suffer.
  • Remember if you are planning on selling your house that each month the project takes means an extra mortgage repayment that needs to be budgeted also.
  • Try to account in your budget for planning permission and Building Regulations application fees.
  • Set some money aside for the costs of decorating the new extension and any new furniture that you may need.

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