The Ultimate Step-by-Step Guide to Building Your Extension

We’re aware that embarking on a construction project can be a daunting prospect. From costing and designing extensions to permitted development and building regulations, this step-by-step guide will provide you pointers and key bits of information, to ensure your project runs as smoothly as possible.

Why move when you can improve?

The UK is rapidly turning into a nation of home improvers. Over the past five years, annual planning applications have increased by 20% to 123,000, as owners look to maximise the potential of their current properties in favour of moving homes.

There are numerous benefits to extending instead of moving house. According to Nationwide the average cost of moving, including stamp duty, legals and removal costs, now amounts to over £9,500. On top of that, you have the hassle and stress of moving combined with the uncertainty in the property market.

Yet, many homeowners are unaware of their properties untapped potential. The addition of an extension or loft conversion can make your current property a more desirable and enjoyable place to live, whilst adding value to its market price. You also have the freedom to design and build what you want, whether it is that extra bedroom, walk-in wardrobe, orangery room or home office.

Step 1 – Prepare a brief

You need to consider exactly what you want from your extension or conversion. There is nothing worse than throwing thousands of pounds into a project and ending up with something that you didn’t really need.

Making changes once works commence on site can be a costly business. Not only will you potentially need to submit a retrospective planning application but the contractor is likely to increase their costs for additional works.

Sit down and think about what will make your life better, whether it is a new ensuite bedroom as you have an expanding family, or a larger open plan kitchen/dining area to entertain friends. Gain inspiration from websites such as Pinterest or by simply walking around the neighbourhood and seeing what other people have done.

You don’t need a complete brief, however, you do need a well defined goal, so that when you sit down with your designer you can give them a concept idea so that they can go away to start working on the proposed plans.

Step 2 – Set a budget

The average build cost for an extension is in the region of £1200m2 for a single storey and £2,000m2 for a two storey. Bear in mind that this excludes fees for a designer to compile the drawings (£600-850), planning application fee (£206), building regulation fee (£450-500), engineers fees (£200-500).

We recommend that when you sit down with your designer (hopefully us!) to discuss your project you give them a budget that you need to work to. They can then look for different design options to reduce the cost if necessary, for example; build the walls in blockwork with a render finish, which is cheaper than the traditional block and brick.

Internal fixtures and fittings can also vary significantly in price. What quality of kitchen do you want? Do you want solid oak doors? What specification bathroom do you desire? All of these factors have an impact on the final build sum, so need to be thought through prior to works starting on site.

Finally, always include a contingency sum of between 10-15% to account for any unforeseen circumstances. Even with the best planning in the world, there will always be some!

Step 3 – Get plans compiled and submit planning application

Once you’ve decided what kind of extension you are after the next step is to appoint a designer who will compile existing and proposed plans, which need to be submitted for planning permission, unless the works fall under permitted development.

Provide the designer with as much information as possible, so that they can go away and compile a couple of draft designs for you to look over. Only once you are happy with the proposals should you submit them for planning.

At ABTO, to save the homeowner the hassle, we always submit the planning application on behalf of them, whilst acting as their agent for the duration of the process.

You are more likely to get planning permission for a single storey extension than a two storey extension, however, if you can get two storey’s approved then it certainly makes financial sense, as you will be getting double the amount of floor space for significantly less than double the cost of a single storey. Your designer should advise on the pros and cons of different options.

At this stage you also need to appoint a structural engineer to compile calculations for any structural elements. Their calculation pack will be issued to the contractor and building control. Most designers have their own engineers who can fulfil this role.

Step 4 – Get quotes from builders

The planning process can take up to 10-weeks from the date of submission through to a decision being made, therefore, to keep the project moving it makes sense to get quotes from builders.

In order to get an accurate price you will need to get Building Regulation drawings compiled, which show the construction details for the proposed works (more on these later). The designer who compiled the planning drawings will do these.

We recommend that you get at least three quotes from reputable builders, to ensure that you are getting value for money. However, be aware that the cheapest price is not always the best. Savvy builders may quote a lower price to get the job before adding in variations once works start on site, so look out for Provisional Sums and Prime Cost sums, which are estimates, not fixed costs.

To avoid this, you need to ensure that your builders are quoting apples for apples. They need to be provided with the full sets of drawings, structural calculation pack, and any specifications that may supplement the proposed plans, such as a kitchen and bathroom designs.

Always get a fixed price from the contractor. Never pay large sums of money upfront (a deposit of 15% is standard practice) and always be wary of paying in cash, as you may have limited evidence in the event of a dispute further down the line.

Don’t expect builders to be able to commence works as soon as you have received planning permission. According to the Federation of Master Builders, 40% of their members currently have a four-month waiting list. A good builder though is worth the wait for your dream extension.

Step 5 – Submit Building Regulation application

By this stage your planning application has been granted and you have a good quality builder ready to start work on your project. Your next step is to submit a full plans building regulation application to either the local authority or an approved inspector. Most builders will charge a small fee to do this on your behalf.

All home extensions need to comply with building regulations, which relate to the structural stability of the building, thermal, sound and ventilation elements and electrical and plumbing works.

You need to get your proposed drawings and engineering calculation pack approved by building control before works commence. If you wait until mid-project you could run into costly trouble should building control require changes to the design.

Once on site, your contractor should liaise directly with the building control officer, who will undertake site inspections at key stages of the build.

Once works are fully completed, a completion certificate will be issued by building control confirming that the work has been carried out in accordance with the building regulations. Keep this document safe, as you will need it when you come to sell the property.

Step 6 – Sort out the legal bits

We always recommend that some form of contract is put in place to protect both the homeowner and the contractor.

Most builders have their own terms of agreement that highlight when payments should be made amongst other provisions. In most instances this should be sufficient, however, it is worth going through it with a fine toothcomb to ensure that no nasty surprises are lucking in the small print.

If you wish to put a formal contract in place then the JCT Homeowner/Occupier is a simplistic contract between yourself and the contractor, suitable for domestic building work.

This simplistic contract, which can be purchased online, specifies the arrangements such as: agreed price, payment terms, project duration and dispute resolutions, rights to cancel, health and safety and stakeholder responsibilities.

Prior to works commencing we recommend that you ask the contractor for a programme of works, which is a reference point that highlights what date certain items of work will be completed by.

The main benefit of a programme is that you can ensure that the builder is on track to complete as they promised. For example, if you notice that the first fix electrics has overran by one week, then you can flag this up before it has a knock on effect on the whole project.

If one project has no deadline, the builder might prioritise other projects, and invariably the finish date will be long after you had envisaged. Talk with the builder and come up with a practical completion date, one that can be realistically achieved.

Step 7 – Post completion

Your dream extension has finally been completed. The contractor has done a cracking job and everyone is happy.

Before you fully sign off the project, we recommend taking a good hour or so to walk around the property to carefully inspect the finish. Make a list of any snagging items, such as: sticking doors, touching up of paintwork, loose sockets, missing silicon etc. Hand this to your builder and ask them to rectify them prior to handing over the final payment.

You also need to get a Health and Safety file from your builder, which may include all or some of the following:

  • As built drawings
  • Building Control sign off certificate (do not release final funds until this is received!)
  • Warranties for boilers, kitchen appliances etc.
  • Gas safe certificate and boiler commission Benchmark form
  • Electrical compliance certificate
  • FENSA certificate for windows

In the months after the works have been completed further defects will crop up. For example; you may start to see some hairline cracks appear in the freshly plastered walls. This could be a common defect caused by thermal movement. Doors may also warp slightly, again caused by thermal movement.

A reputable contractor will return and make these defects good. However, you may wish to withhold a retention of say 2.5% of the contract sum for a period of 3-months to protect your own interests. This will need to be agreed prior to works commencing though.

Thinking of building an extension?

We are offering a free consultation to discuss your project, whilst providing advice on the different design options that are available to you.

Simply send us an email or give us a call, and we will arrange a meeting to discuss things further.

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(t) – 07525 416 310


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