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How to Design & Plan a House Extension

Home extensions have gotten increasingly popular recently. With a property market that’s slowing down, many are choosing to add space and value to their home through conversions and extensions. If a house extension is something that you’ve been thinking about, this post is going to run through the design and planning process.

Things to Consider Before

Extensions need some serious thought, good planning and better execution. There are a few questions that you need to ask yourself before you get started. There’s four things in particular that you need to consider that we will expand on in more detail. These are:


What do you want from your extension? Are you looking to add an office? A bedroom? Increase the size of your kitchen? Think about how the extension will relate to the rest of your home. This is particularly important for considering the design. You need to know whether you’re extending sideways, upwards or downwards. What will you gain from each option? Is it a good investment? Will it add value to your home? Considering how you’ll use the extension also allows you to adapt the design to your needs.


You didn’t need us to tell you this and is probably the first thing you considered, but you need to set a budget. We’re going to take a closer look at the costs of an extension later on in this post but there are certain fees you need to account for. These include architect’s fees, planning applications, potential insurance premium increases and VAT to name a few.

Costs aren’t the only thing you have to consider, but also what the eventual added value of the extension will be upon completion. If the cost is less of an issue to you and you just want to extend, that will require you to think a bit differently if you hope to add value to your home. This is important because with the correct due diligence you may discover that it makes sense to invest more into the project if it results in greater returns further down the line.


Depending on where you live, there will be different rules and regulations to abide by. You may need planning permission, you may not. Conversions and some single-storey extensions, depending on the size, can often forego the need for planning permission. Make sure you’re aware of all the permissions and permits you may need.

Your house may also be on Designated Land. You can check with your local authority whether your property falls under that category. Local authorities will have a Local Land Charges department which can provide that information for you, sometimes at a cost.


An extension is not something that is or can be rushed. It can take months just to obtain all the right permits and permissions let alone build it. You will need patience. Consider how the construction work will conflict with your personal life. Most people stay in their home during the process of an extension, but this depends on the situation.

Consider also the disruption to your neighbours. It’s a good idea to keep your neighbours up to speed on your plans, out of courtesy if nothing else. If you are in a semi or terraced house, work on your home could cause structural damage to theirs so communication is important.

Project Manager

Designing a House Extension

If you’ve weighed up all the options and have come to the conclusion that a house extension is definitely the right course of action for you, the prospect of designing it can be quite overwhelming.

You don’t want to make any errors that will lead to you racking up additional costs that eat into your budget. Don’t think you have to cede all creative input by drafting in some professional help. Architects want your needs to be realised with their design. Architects have the benefit of years of experience in designing extensions which you don’t so they can advise on how you can bring your ideas to reality.

When consulting architects, you need to create a brief of what you want from your extension. Try not to think exclusively about the space you’re adding, draw up your brief around what you want as well as what problems need resolving. Your brief should include:

  • Budget specifications
  • Essential details e.g. more light or new bedroom
  • Current issues with the house
  • Family requirements e.g. any children or disabled access
  • Taste detail e.g. specify what you don’t like to see in a house and what you do
  • Any other important information such as living on Designated Land or in a listed building.

Make sure your brief is detailed, with particular attention to how you express your budget. Don’t just give a figure of what you’d be willing to spend. Specify a maximum and an allocated budget so the architect can get closer to the mark of what you want for the right price.

The more detail you include, the less likely you are to face barriers to completion. If the architect has already been able to consider as many factors as possible, this will make the route to completion more painless and direct.

When choosing your architect, you may also want to consider at this stage your construction company. They will work closely together throughout the process as often your architect will continue to be on hand to make sure things are running smoothly. It’s important the communication between the two is clear so your designs are carefully realised. You may want to choose a construction company who can offer an all in one service as the transition often runs more smoothly.

Choosing a Type of House Extension

Now, you probably have an idea in your head of what your extension will look like. We’re going to look at some different types of extension in case there are any that you may have overlooked and could satisfy your requirements.

Front, Rear & Side Extensions

If extending outwards then you are going to be opting for one of these. Rear extensions tend to be more common as it is easier to extend into your own garden space. Front extensions are harder to come by because there are more conditions that depend on it being allowed to go ahead. If you don’t want to compromise on garden space and a front extension doesn’t look likely, then extending out to the side is a good option. The extension you opt for often depends on the type of property you have and the space surrounding it.

Over Structure Extensions

These can be carried out over existing structures such as a garage.

Additional Storey

You can add a whole new level to your home. In built-up areas this is obviously much harder to do than say, but excavating a basement if you live in a single-storey dwelling it will add a ton of value.

Choosing a Roof for your House Extension

Felted Flat Roof

The cheapest and easiest of the lot, the felted flat roof is a popular choice. If you’re extending outwards, particularly to the rear, a felted flat roof is your most cost-effective route. It can adapt to any shape of extension and a flat ceiling makes it easier to install lighting. Unfortunately, they are more prone to leaks and have a lower average lifespan than other roofs.

Inverted Flat Roof

This is the almost the same as a felted flat roof but includes another layer of material on top of the waterproofing and insulation to turn it into an external space. This is usually done with paving stones and has the added benefit of becoming another layer of protection against the elements, increasing its lifespan. The drawbacks to this is that there are planning permissions required to turn it into an external space, there may be overlooking issues and other costs such as constructing a barrier around the perimeter and so on.

Glazed Roof

The natural light option. If part of the aim of your extension was to try and get more natural light into your home, a glazed roof could be for you. If your extension is over a workspace or your kitchen, this is a really stunning option. Being able to catch all that natural light can transform your living space completely. You will have to sacrifice on insulation and there are also extra planning considerations.

Pitched Roof

This is more if you’re adding an additional storey or extending at the front but the pitched roof is the classic option. Design-wise, it leaves you some options such as whether to have a high ceiling or to create some loft space. This is also favoured by planners, it meets requirements easily and they tend to look more like a natural extension. This option is more expensive due to the materials and added complexity to its installation but could really boost your space.

Choosing Windows and Doors for your House Extension

Now you have your roof sorted, you also have to consider your windows and doors. There’s loads of different types and plenty of potential to get creative. First up, we’re going to look through some types of window:

Plastic Windows

We’ll kick things off with the cheapest option again, plastic windows. Favoured for their sheer simplicity, cost and low-maintenance, there’s even been a shift to using recycled plastics to decrease their impact on the environment. However, they’re not the most aesthetically pleasing. It depends largely on what you’re working with. The price will depend on quality, but they require essentially no maintenance so they’re a good go-to option.

Metal Windows

If you’re going for a more modern, sleeker look, metal windows could be your thing. They are equally as low maintenance as plastic windows but look much more up to date. They’re slightly more expensive but look amazing as roof windows and can normally be found in any colour.

Wood Windows

Wooden windows have their own subcategories: hardwood, softwood and composite. If you’re trying to achieve a timber look but you’re on a budget, then softwood is the way to go. It will need to be repainted every few years or so but is generally low maintenance. Hardwood is obviously more durable than softwood and emanates a more authentic look and feel of wood. It can be much more expensive than softwood in some cases so shop around. Composite windows are usually made of timber and metals, they’re a good triple-glazing solution if you want to bump up your insulation. It will have a timber look on the inside and offer the protection and low maintenance of metal windows on the outside, they are one of the most expensive options however.

For doors, we are going to assume that you’re looking for external doors. It’s likely that you’ll choose a door that’s consistent with the rest of the house if it’s a single access point. You can get plastic and wooden doors and they pretty much have the same pros and cons as the windows.

However, installing patio doors can be where you see your biggest transformation. If extending out to the rear or just into the garden, some bi-folding or sliding doors that go from floor to ceiling can revolutionise your space. With some clever planning and designing, you might only extend out by a few metres but can make that space feel like a whole lot more.

Making your patio flooring consistent with your kitchen flooring will make the room look and feel bigger. The doors will allow light to bathe the room. If you opt for a glazed roof as well, the lines between outdoors and indoors will be blurred and it would be a great space for entertaining. This may also negate the need for any windows so although patio doors and a glazed roof are more expensive options of roof and door, if it saves the need to install windows it can be worth it.

Garage Extension

Cost of a House Extension

House extensions come in all different shapes and sizes, with a whole load of different factors to consider. This is why we recommend making your brief as detailed as possible, so your architect or builders can be as accurate as they can with their figures right from the off. But we’ll look at some average prices.

For a single storey extension, the most common type, you’re likely to be looking in the region of £1,000-£1,500 per m2. So, for a 5 x 7m extension you can expect a ballpark figure of £43,000. When you add on VAT and professional fees that figure can swell up to £57,000. In some areas in the South East and in many parts of London, this cost rises to between £1,500-£2,000 per m2. If it’s a double storey extension, our £57,000 figure from before will go up to around £85,000.

Extensions can add extra value to your home however and if you’re planning on moving in the future this is something to consider. This is something you’d have to comb over with an estate agent. It doesn’t hurt to get an estate agent’s advice on what will add value to your home and factor that into your plans.

Those figures really are ballpark figures, there’s so much to consider. The soil type may mean that you have to go for more expensive building methods to accommodate it. If you have to tear through pipework or drainage that will cost you more. Adding a kitchen will cost about £10,000 and a bathroom is likely to set you back £5,000.

Planning for a House Extension

One of the most tedious and frustrating parts of building an extension is the planning permission and permits that are required. There’s a good reason why we have them, so areas and streets can keep their character and of course so that people don’t go wild with their home improvements.

If you’re converting an inside space like your basement, this normally does not require any permission unless you’re making structural changes. Most extensions may require you to obtain some permits. Here are some criteria that avoids having to obtain planning permission:

  • You are permitted to build an extension to the rear of up to 8 metres as long as it is not higher than 4 metres or the height of the existing property.
  • You are permitted to extend by 3m if it is a double storey extension.
  • Your extension must not end up with more than 50% of your garden being covered.
  • You must not extend beyond the original building line of your home.
  • Double storey extensions have to be further than 7 metres from the rear boundary.
  • You must construct your extension with the same or similar materials as the existing building.
  • If extending to the side, only single storey builds are permitted that are less than half the width of your home.

If the extension you want can fit within these criteria, then you can save time and money. You will want to go over the details with your architect as there may be ways to tweak your plans that result in the work getting underway quicker.

Hopefully, this post has given you everything you need to go ahead and start planning for your extension. The main points to take away are to seek professional advice and consultation and to be as detailed as possible when sending out a design brief. Here at Berkeley Build, we offer advice and building services and would be happy to listen to your ideas and plans to see what we can do for you.