Beginner's Guide- Building an Extension

Updated: Dec 3, 2018

Extending is a popular way to add space without having to move. We explain everything you need to know from planning permission to costs, design work and much more.

st. Alban's House - by INTEGRATE.HOUSE

What Do You Need to Consider Before Extending?

It is worth thinking about your extension from an investment point of view. And as with all investments, you will want to see a return. Will your added space add value to your home? Normally it will, but there will be a ceiling price for properties in your area and you need to be mindful of the stage where it does not make economical sense to add an extension.

Also, if you are going for a single storey extension, it is worth seriously considering what benefit you could get from a two storey extension – if you are going to be building anyway.

There are also practical issues to consider that are not directly concerned with the construction process. Access is a good example. If you add to your accommodation, will it mean more cars will need to be parked on the drive? If you have no drive then the lack of off-street parking might be a reason for the refusal of planning permission.

Similarly, if your house is in a terrace do you have rear access for the unloading of building materials or if not, will you have to bring everything from beams to blocks, and girders to guttering through the house?

Other important aspects to consider before you get to the stage of getting your plans drawn are matters like:

  • soil conditions on the site

  • services

  • surrounding trees

  • any history of flooding

  • rights of way.

How Much Does an Extension Cost?

The cost of your extension will be affected by many variables, from soil type (which impacts the foundations); whether you are building a single or two storey extension; what the extension will be used for; to how much glazing you plan to specify.

Depending where you are in the UK, for a straightforward extension you should allow around £1,000–3,000/m². Remember that the standard of specification you choose will have an enormous influence on the build cost. Experienced renovator Michael Holmes, says a single storey extension will cost the following per/m²:

  • Basic quality £1,000 to £1,680

  • Good quality £1,800 to £2,200

  • Excellent quality £2,200 to £3,000

A two storey extension will not cost much more per square metre because, aside from the extra interior fixtures and finishes, you are only adding walls and floor joists — a roof and foundations are required whether your extension is single or two storey.

How to Finance Your Extension

There are several options when it comes to borrowing money to finance your extension. However prudence is the watchword — if you borrow more than you need you might do unnecessary things in the course of the job and have difficulty paying it all back. So always be conservative.

In an ideal world, we would all finance our projects using our savings, but with recent times having offered poor interest rates, the financial landscape has been one that favours borrowers over savers. If you need to borrow the money, your best options are:

  1. Remortgage or secured loan: If you are borrowing more than £25,000 you may need to remortgage your home or take our a secured loan against your home. Many building societies offer what is known as a Home Improvement Loan of up to £200,000.

Will I Need Planning Permission for my Extension?

Planning consent may or may not be required for your proposed extension. Under the Permitted Development Rights system a large number of home extensions can be built without the requirement of planning permission.

  • You can extend a detached dwelling by 8m to the rear if it’s single storey or 3m if it’s double

  • There are height restrictions. A single storey extension not being higher than 4m in height to the ridge and the eaves, and ridge heights of any extension not being higher than the existing property

  • Two storey extensions must not be closer than 7m to the rear boundary

  • It must be built in the same or similar material to the existing dwelling

  • Extensions must not go forward of the building line of the original dwelling

  • Side extensions must be single storey, maximum height of 4m and a width no more than half of the original building

  • In Designated Areas side extensions require planning permission and all rear extensions must be single storey

  • An extension must not result in more than half the garden being covered

  • You can only do it once and the original building is either as it was on 1st July 1948or when it was built. In Northern Ireland it is as it was built or as it was on 1st October 1973

You should bear in mind that if your house is in a Conservation Area or a National Park, the amount of work one can do under Permitted Development is usually reduced.

Your local authority has the power to remove Permitted Development Rights if it feels the character of the area wll be threatened by any new work. If in any doubt, you should check with your local authority planning department.

If you are doing an extension that will need planning permission, it is wise to pop into your local planning office to find out informally what might be permitted — especially if you are planning anything out of the ordinary. It is always wise to research the local planning policies so that you will be aware from the start that an uphill struggle awaits you if you plan anything too exotic in the area where your house is situated. The safest and most convenient way is to work with professionals who are familiar with this process and can assist you along the way. Feel free to contact our team of experts at INTEGRATE.HOUSE if you have any question.

Listed Buildings

All alterations to listed buildings, including internal ones, require consent and it is a criminal offence to alter a listed building without this consent. With a listed building the planners will always regard the existing property as more important than what you are proposing to add to it. Any extension will therefore have to respect the flavour, appearance and historic material used in the construction of the original house.

Depending on the size of the extension you propose, you might need planning permission as well as listed building consent to make your alterations.

Local Authority Grants

Unless your house is in an exceedingly poor state or is listed Grade I or Grade II*, or you are disabled, you are likely to have great difficulty in obtaining a grant for your work. However some organisations will offer help to maintain a period property (usually only to bring it to a basic standard of living to avoid demolition), so enquire with your local authority.

Building Regulations

Regardless of whether your new extension does or does not require planning permission, it will need Building Regulations Approval. Building regulations are rules approved by Parliament laid down to ensure the mimimum design and construction standards are achieved. These cover all manner of subjects such as fire and other forms of safety, insulation, the drainage system, and access.

Building control officers do not supervise work on your behalf. Their role is to ensure the minimum standards of the building regulations have been adhered to.

To meet Building Regulations you either:

  • Send what is called a Full Plan Submission to your local authority. In this case you pay a fee and the building inspector visits the site at the various stages of the build and inspects the work as it proceeds

  • Submit a Building Notice. This is a statement in which you inform the council that you will be complying with the regulations in building your extension and gives the building control department 48 hours notice of your intention to start the work. Surveyors will come and inspect the work at various stages and will advise you of any problems

The second method carries an element of risk because you do not have the benefit of an approved plan to work to and the building control surveyor may only know after you have contravened a regulation requirement. It could therefore prove to be an expensive way to build if problems are discovered that have to be rectified.


Getting a warranty for your extension is advisable as it will cover you for things such as structural defects or faulty workmanship. Should the worst happen, the builders who did the work will need to return to put right what has gone wrong.

How to Design an Extension

Some people design their own extensions, using structural engineers for advice. Others use a design and build company who can take on the whole project.

The advantage of hiring a professional to do the design work, is that they might be able to visualise the space in a way that you had not thought of due to overfamiliarity with the existing layout of your home. It is also advisable to work with a professional designer with experience in the area if you are extending a listed home or a home in a Designated Area. Often using professionals will help you find the optimum lay out. It is important to work with designers or practices who specialize in house design and residential projects in the UK. Specialists designers and architectural practices have an extensive knowledge and experience in every aspects of this process. They are able to advise you on possible design, help you finish on time and on budget, maximizing the return of your investment (sometimes up to multiple times more than their professional fees), and advise you on choosing the right materials and suppliers.

Contact INTEGRATE.HOUSE team to explore how we might be able to help you with your project.

Building an Extension

Who Will Manage the Work?

Early on, you will need to decide who will manage the build of your project. If the work is carried out by a design and build company, this will be managed for you, but if you have designed it yourself, or used an architect, you need to find a main contractor to pick it up from here. Alternatively, you might choose to manage the build yourself hiring subcontractors for each stage. Instead, you could hire a project manager to take on this role for you.

Using a main contractor

With this self build route, you employ a main contractor to run the building site on a day-to-day basis. This will usually involve the main contractor being responsible for:

  • organising a smooth flow of labour onto the site when necessary (and paying them directly)

  • dealing with the unloading of deliveries

  • organising warranty and Building Regulations inspections

  • running the site itself (e.g. health and safety, toilet facilities and so on)

  • working from the design plans

  • The main contractor might also be responsible for ordering materials and ensuring they are on site when necessary

Your Input

Firstly, you will need to choose a main contractor, going out to tender to secure quotes. (You’ll need to provide your Building Regulations’ plans and as much detail as possible in order to receive accurate quotes to compare on a like-for-like basis.)

Once you’ve appointed your contractor, it can also be a very good idea to visit the site at least once a week to check on progress, ensure that the drawings are being followed, and that the main contractor has everything they need. Your main role, aside from payrolling the project, will be to ensure you’re at the end of the phone when needed — which will be often. Communication really is key.

You will also need to make sure the main contractor is paid regularly and promptly — they’ll be paying for the subbies, but it’s your responsibility to ensure the cashflow is regular.

If you have any further question or if you are thinking of renovating or extending your existing house email us at or call me (Mehran Gharleghi) at 07517477724.

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