Four steps to plan and design a house extension

Extending your property is a good way to get extra space, add value to your home and improve your lifestyle. For someone that has never been involved in the construction industry, building an extension is a big undertaking.

There are numerous steps involved, as well as lots to think about and do. However, as with so many things in life, with the right advice and support building an extension is far easier than you think.

All you have to do is to break the process down into smaller, more manageable steps, which is what we have done for you below. Following our advice will ensure that the extension you build will be 100% legal, well built, fit for purpose and affordable.


Why use an architect?

Our first tip is to hire an architect to help you with the design and planning process. A good architect has the necessary skills and experience to guide you during the design phase and help you to end up with the right type of extension for you and your family.

For most extensions, you will need to seek planning permission. Part of that process is submitting plans to the council. Your architect will draw up those plans as part of their service.

Your builder will also use the plans produced by your architect. They will use them as a guide to make sure that the finished extension is what you want and have paid for.

Finding the right architect for you

Therefore, your first step is always to find, and engage, a good architect. Getting them involved at an early stage is the best way to save you a lot of wasted time and unnecessary hassle.

Here are a few tips to help you to find the best architect for your building project:

Properly qualified

The first step is to make sure that any architect you are considering hiring is properly qualified. Unfortunately, there are some people out there claiming to be architects that are not. You need to avoid using these people. They do not understand the regulations, are not insured and do not offer reliable advice, so you do not want to waste money hiring someone like that.

In the UK, The Architects Registration Board (ARB) maintains the register of properly qualified individuals and firms. You can use their website to get a list of the professional architects that work in your area.

The right experience

It is important to use a firm of architects that has the right type of experience. You need a company, like ours, that is used to helping private clients to plan their home extensions. Ideally, they will have experience of planning the type of extension you are interested in building. For example, if you want to build using materials with good green credentials, hiring an architect that has worked on that type of project before makes sense. Likewise, if you are trying to extend a building in an environmentally sensitive area it is a good idea to hire an architect that has worked on similar projects.


Easy to communicate with

When looking for an architect you need to look for someone that is easy for you to communicate with. Good communication is crucial. You need someone you feel comfortable with that understands what you are trying to achieve. It is important to look for an architect that listens well and explains everything in a way that is easy for you to understand.

Creating the brief

You now need to create your brief. This an important step, if your brief is clearly written, it will be far easier for your architect to come up with a suitable design.

This document tells your architect what you want. Writing the brief will also help you. The writing process will help you to focus on why you are adding the extra space to your home. It will make you list out what you need to include in the finished building and make sure that nothing is missed.

Here are the main points that you need to cover in the brief:

Your aims

In this introductory section, you briefly outline why you are building your extension. For example, if your primary aim is to provide more bedrooms for your children that can later become part of a granny annex you need to say so in the brief. If you are primarily building to add value to your home put that down as your aim.

It is likely that you have several aims. Ensure that each one is included in the brief and try to list them in order of importance. Place the emphasis on your most important aims.

Your budget

Your architect needs to understand how much cash you have to spend. In this part of the brief, you should state how much you are hoping to pay for the extension and explain what the upper end of your budget is.

We cover working out how much you have to spend later on in this article.


How the extension will be used

You need to explain how the additional space you are adding will be used in both the short and long-term. For example, if you are planning to have a bedroom double as office space, your architect will make sure that the design includes a phone connection for that room.

If the extra bedroom you are planning to use for your child is to be adapted, later, to lease out to private tenants, your architect will incorporate space to make it viable for you to install an en-suite at a later date. For example, they will make sure that the window is placed to leave enough space for you to install a shower and toilet in a corner of the room should you want to.

Your design style

Here you explain what style of extension you want. In many areas, local building regulations require that extensions blend in with the style of existing buildings. As a result, there are often limits to what you can do, style wise.

Your architects will advise you about this, however, at this stage you are putting together your ideal, so tell your architect what you would really like. They will work, within planning law, to come up with a design that gets as close to your vision as possible.

The finish and design ethos

In this section you also need to outline any special concepts you want to be built into your extension. For example, if you want your extension to be built using eco-friendly materials, this is where you say so. If you want a high-tech extension that incorporates certain features, include that fact here.

Go through your brief several times and take your time. Remember that this document is the foundation of your working relationship with your architect. Your brief is their initial reference point. It informs which direction they go in when designing your extension, at least, to start with.


The initial meeting

You will use this brief extensively. It is important to have it ready for your first meeting with your architect.

Setting your budget for your extension

As we mentioned earlier in your brief, you need to outline your budget. This is an important reference point for your architect. How much money you have to spend informs many of the decisions they make while designing your extension.

For someone who is not involved in the building trade setting a realistic budget can be difficult. It is hard to know where to start, which is why we have devoted some of this article to showing you how to work out your budget.

Unavoidable building costs

The first step is to establish how big your extension needs to be to meet all of your needs and aims. This gives you the square metre figure you need to establish what the minimum cost of your extension will be.

Per sq footage building costs

As of 2015, it costs around £1,200 per sq metre to build a single storey extension, in most parts of the UK. Therefore, the basic building costs for a 10 sq metre extension would be £12,000.

If you want to put an extra storey up, the per sq metre build cost is around £1,800. This means that a double storey extension with a 10 sq metre footprint will cost around £18,000.

However, it is important to understand that the per square metre costs vary depending on where you live in the UK. This is mainly because the cost of labour and running builder’s premises varies from area to area.

If you live in the South East, your building costs will be higher. Typically, you can add around £100 per square metre to your costs. Therefore, the basic building costs of a 10 sq metre single storey extension in the South East will be £13,000 rather than £12,000. Those who live in Greater London need to add around £300 to the basic per sq metre cost.


Professional fees

You now need to add between 10% and 15% to the basic building costs for professional fees. This covers the cost of your architect, building regulation inspections and, if needed, the services of a structural engineer.


Add the sq footage cost to the professional fees to get your pre-Value Added Tax (VAT) figure. Now add the VAT in to get the basic cost of your build. Currently, the amount of VAT you have to pay on a home extension is 20%.

Once you have added all of these figures together you have the cost of constructing a simple timber frame extension, putting a basic roof on it, plaster boarding and installing a basic floor. It also includes the cost of basic windows.

Fixtures and fittings

This figure does not include any fixtures and fittings. Therefore, you will also need to budget for the cost of installing things like kitchens, bathrooms and fitted wardrobes.

You can get a fair idea of how much to budget for these by going online and looking at the type you are planning to install. Remember to factor in the cost of installation.

Extras and hidden costs

Bear in mind that this is for a basic building built out of ordinary materials. If you want a wall that is made of triple glazed glass, it will cost much more than one built out of brick. Therefore, if the use of high quality, expensive building materials is an essential part of your extension you need to factor that into your budget too.

If your site has restricted access, it will cost more to have building materials delivered, so your build costs will be higher. The same is true if a lot of soil needs to be removed to allow the foundations to be put in, or the drainage will need to be moved.

Unforeseen issues

In addition, it is always wise to have a little spare cash available for unforeseen eventualities. If you have ever watched a building programme on TV you know that things can, and do, go wrong, so it is wise to leave a little wiggle room in your budget. That way if something unexpected crops up you will have it covered and be able to afford to finish your extension.

How much you can afford

Now you know how much your building work is likely to cost, the next step is to check that you can access the funds. This is essential if you are planning to re-mortgage to pay for your extension. There is no point in going through all the hassle of writing a brief and meeting with an architect to discuss an extension that you cannot actually afford to build, at least not yet.

Does it still make financial sense to build the extension?

At this stage, it is a good idea to ask yourself if it makes financial sense for you to go ahead with your building plans. If your main aim is to add value to your property you want to be sure that the bill for your extension will not be higher than the amount added to your home.


Finding a builder

Once you have met with your architect, settled on a design and are sure you can secure the finance you are ready to find a builder. You need a reliable company, with a good reputation and experience of building the type of extension you want.

Your architect will be able to help you with this step as well. They work with lots of builders, so know which ones are reliable and are good at what they do.

The right experience

However, you should also do your own research. Most good builders will provide you with references if you ask them to.

If you can, look at work they have already done. Look at their website too. You are looking for someone who has experience of building the type of extension you want to be built.

Are they insured?

It is important to make sure that the builder you use has proper insurance. That way, if something goes wrong you will not lose out financially.

Get a quote and a guarantee

Once you think you have found a good builder get a detailed quote from them. Ask them to include details of their guarantee.

Draw up a contract

You now need to draw up a contract that outlines the work that will be carried out. It should also include a staged payment plan and timescales. Make sure that the last payment is only due once snagging has been done, and the extension has been properly finished.

Follow these four simple steps and you will end up with the extension you dream of. Giving your family the extra space they need and adding value to your home at the same time.

Posted by Adam Lloyd
February 15, 2016

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