Planning a House Extension: Where Do You Start?

When you first start thinking about extending your house, it can be difficult to know where to begin. Do you work out a budget, decide how big the extension should be, speak to an architect, start drawing layouts or find out what planning permission you might need?

Having been through it ourselves, we think the best place to start is to work out what you actually need the extension to achieve. The dimensions and design can develop from there, and they will probably change as you learn more about the house, the site and what everything is likely to cost.

We bought our house knowing that we wanted to extend it. We had a good-sized garden and a fairly clear idea of what we wanted: a larger kitchen, an open-plan dining space and an additional bedroom.

What we didn't know was how large the extension needed to be, how much it would eventually cost or how many decisions would change along the way.

The main thing we learned was that very few decisions exist in isolation. Changing the size can affect the cost, structure, drainage or planning position. Moving a wall can put you closer to a boundary, tree or underground drain. Something that appears to be a small alteration on a drawing can affect several other parts of the project.

Start with what isn't working

Before deciding on dimensions, think about what you want to improve in the existing house.

For us, the kitchen was too small for the way we wanted to use it. We wanted more kitchen space and storage, but also somewhere where the kitchen and dining area worked together. We also wanted another bedroom.

Those were our starting requirements.

Once you begin looking at plans, however, it is easy for the wish list to grow. A slightly larger kitchen would be useful, another metre around the dining table would be nice and perhaps wider doors would look better.

Then comes a phrase that can add a considerable amount to an extension budget:

"While we're doing it..."

Some of those decisions make sense. There are jobs that are much easier to do while builders are already on site and parts of the house are being opened up. The problem is that a collection of individually reasonable additions can turn into a much larger project.

It helped us to keep coming back to the reasons we wanted to extend in the first place.

Separate what you need from what you want

It is worth making two lists before getting too far into the design.

The first is what the extension genuinely needs to achieve. The second is everything that would be nice to have if the space and budget allow it.

Our essential list was fairly straightforward: a larger kitchen, an open-plan dining area and another bedroom.

Everything else had to justify the additional space or cost.

That doesn't mean being overly restrictive. If you are already carrying out major building work, there will be occasions when spending a little more makes sense. It simply helps to recognise when something has moved from being a requirement to being a preference.

How much space do you actually need?

For some houses, the garden or shape of the plot will put an obvious limit on the size of an extension. That wasn't really our problem because we have a good-sized garden and could extend while retaining plenty of outside space.

For us, cost was a greater constraint than the size of the plot.

Having enough land to build a larger extension doesn't mean that building the largest one possible makes sense. We needed to decide how much additional space would make a meaningful difference and how much we were prepared to pay for it.

This is also the point where it helps to stop looking at the plan as an empty rectangle.

Plan around how the room will actually be used

If you want a kitchen island, for example, you need enough space not only for the island itself but also around it. Cupboards and drawers need to open, people need to be able to move comfortably around the kitchen and, if you want stools at the island, there needs to be enough room for someone to sit there without blocking the route behind them.

The same applies to a dining table. Allowing for the table is one thing; allowing enough room to pull the chairs out and walk around it is another.

We found it more useful to put the furniture and fittings we actually wanted onto the plan rather than looking only at the overall room dimensions. Could the dishwasher be opened without blocking the main route through the kitchen? Was there enough room behind the island seating? Could people move around the dining table comfortably?

Sometimes another half metre genuinely improves the way a room works. In other cases, changing the layout achieves more than making the whole extension bigger.

Set a budget before the design runs away with you

You don't need an exact construction cost before you start sketching ideas, but you do need some idea of what you can realistically spend.

Extension prices are often discussed as a cost per square metre. That can be useful for an initial estimate, but we learned not to assume that increasing the floor area by a certain percentage would increase the final cost by exactly the same amount.

Some costs barely change when an extension becomes slightly larger, while others can jump because the change requires a different structural solution, more complicated drainage, larger glazing or something else that wasn't needed before.

Our own extension illustrates this quite well.

At one stage we were considering a depth of around 5 metres and eventually decided to extend to around 6 metres. At the time, another metre didn't seem particularly significant because we were already paying for foundations, walls and a roof.

Increasing the span eventually affected the roof structure and the type of joists we needed.

The extra metre wasn't simply another metre of bricks and flooring.

That experience changed the way we looked at alterations to the design. Rather than only asking how much a change would cost, we began asking what else would change because of it.

Check the house before deciding what you can add to it

Before becoming too attached to a design, find out a little more about the house itself.

Look at its planning history and whether previous owners have already extended it. What you see today isn't necessarily the original house, and previous alterations can affect what you are able to do next.

It is also worth checking whether there are any particular restrictions affecting the property. A listed building, conservation area or property where permitted-development rights have been restricted needs different consideration from an otherwise similar house without those constraints.

Your title documents may also contain things that have nothing to do with planning permission, such as covenants, rights of way or restrictions over parts of the property. If the house is leasehold, there may also be a requirement to obtain the freeholder's consent.

You don't need to become an expert in planning or property law before drawing anything. The aim is simply to find out whether there is something fundamental that could influence the project before you start spending money developing the design.

Look at the whole plot, not just the empty lawn

A garden can look like an empty area ready for an extension, but there can be plenty to consider above and below ground.

Boundaries and neighbouring buildings matter, as do trees, changes in ground level and the amount of working space around the proposed extension. Underground, there may be foul drains, surface-water drains, public sewers, water pipes and other services.

The rainwater from the new roof also has to go somewhere. If a soakaway is needed, it needs a suitable position in the garden, so that is something to think about before the extension occupies the place where you might otherwise have put it.

Friends of ours discovered water-company pipes while their extension was already under construction. Work was effectively held up while they went through months of correspondence and applications before the pipes could be moved.

Nothing visible from the garden suggested there was a problem.

That experience stayed with us because it showed how something you couldn't see could have such a large effect on the project.

We will look separately at foul drainage, public sewers, surface water and soakaways because there is much more to the subject than belongs in an introductory article.

Think about daylight and which way the house faces

The direction the house faces can also influence the layout.

An extension changes the relationship between the original house and the garden, particularly if you are creating a large open-plan space. Think about where the sun is at different times of day, which rooms already receive good natural light and whether extending behind them might make the original part of the house darker.

More glazing isn't automatically the answer. Large areas of glass can affect heat loss, overheating, privacy and the amount of usable wall space inside.

You don't need detailed calculations at this stage, but it is worth looking at how the proposed rooms relate to the actual orientation of the house rather than designing the layout in isolation.

Look at the existing house as well as the extension

It is easy to concentrate entirely on the new space, but eventually it has to connect to the house that is already there.

The opening between the old and new parts may need structural support. Existing floor levels can influence how the two spaces join. The position of drains, soil pipes, boilers and electrical equipment may affect the layout.

There may also be existing problems or ageing services that are easier to deal with while major building work is taking place.

If you are adding substantially more living space, it is sensible to think about whether the existing heating, hot-water and electrical systems will be suitable for the finished house.

You don't need all of these answers before the first design is drawn, but knowing about potential issues early can prevent an otherwise attractive layout becoming unnecessarily difficult or expensive to build.

Can you actually build it?

The extension may fit comfortably into the garden, but it is also worth thinking about how it will physically be constructed.

Can a mini-digger get into the garden? Where will a skip go? How will soil from the foundations be removed? Where will bricks, blocks, insulation and other materials be stored?

Large steels, roof joists, windows and doors also need a route to where they will eventually be installed.

If there is good side access, much of this may be straightforward. If the only access is through the house or along a narrow passage, the same project can be more difficult and potentially more expensive.

The finished extension has to fit the site, but so does the process of building it.

Think about how you will live through the work

This probably isn't the first thing most people consider when drawing an extension, but it can have a significant effect on the budget and how disruptive the project becomes.

Our project involved the kitchen, so at some point the existing kitchen would no longer function as a normal kitchen. If a project affects the main living areas, bathrooms or access through the house, think about what temporary arrangements might be needed.

You may be able to stay in the house throughout the work, perhaps using a temporary kitchen elsewhere. For a more extensive project, there may be a period when moving out makes more sense.

Alternative accommodation costs money, but living in a building site has its own drawbacks.

You don't need a detailed construction programme at this stage, but it is worth understanding how much of the existing house will be affected.

Think about planning early

Planning rules obviously need to be considered before a design is finalised, but we wouldn't start by simply drawing the largest extension that might be allowed.

First decide what you want the space to achieve, then establish what planning rules apply to your particular house and location.

We are in Wales, and one thing we soon discovered was how much information online is written specifically for England. The rules aren't identical, so dimensions or permitted-development advice from an English article or YouTube video shouldn't automatically be applied to a house in Wales.

Previous extensions, the type of property and where it is located can also affect what is possible.

Planning permission and permitted development deserve an article of their own, so we will look separately at the differences between Wales and England and what needs checking before an application is made.

Planning permission and Building Regulations are different

This is another distinction worth understanding before the project gets too far.

Planning is largely concerned with whether the development itself is acceptable. Building Regulations deal with how it is constructed.

An extension might not require a planning application but still need Building Regulations approval.

Structure, foundations, insulation, ventilation, drainage, electrics and other technical matters become increasingly important as the project develops.

At this first stage, you don't need to know every Building Regulation. You just need to understand that obtaining planning permission doesn't mean every technical detail of the extension has been resolved.

Before you pay someone to draw it

You don't need to solve every technical problem yourself before speaking to an architect, architectural technician or designer, but it is worth doing some basic homework first.

Thinking about the size you actually need, your likely budget, previous extensions, boundaries, drainage, trees, access and the way you want the finished rooms to work can save both time and money.

There is little benefit in paying someone to develop a detailed design only to discover later that a sewer runs through the proposed footprint, the soakaway has nowhere sensible to go, access for construction is extremely difficult or the dimensions create a problem you hadn't considered.

You may still need professional advice to work out the solution, but identifying possible issues early means that advice can be used to shape the design rather than to undo work you have already paid for.

That doesn't mean arriving at the first meeting with a finished plan. In many ways, it is better not to.

What is useful is arriving with a clear idea of what you need the extension to achieve, what you can realistically spend and what constraints you already know about.

Who should you speak to first?

Once you have worked through the basic questions yourself, you should have a much clearer brief to take to an architect, architectural technician or designer.

A good designer may come up with a better solution than the one you originally had in mind while also spotting planning, structural or practical issues that need further investigation.

A structural engineer may become involved where beams, roof structures, foundations or other structural elements need calculating. Building Control becomes important as the technical design develops, while drainage or other specialists may be needed if the site presents particular problems.

The aim of doing some homework first isn't to replace those professionals. It is to make better use of them and avoid spending money developing ideas that could have been ruled out much earlier.

Where we would start now

If we were beginning again, we would start by deciding what wasn't working in the existing house and what the extension really needed to achieve.

From there we would set a broad budget, put the furniture and kitchen layout into the proposed space, check the history of the house and look at the main constraints around the site.

We would want to know where the obvious drains and services were, where rainwater might go, whether there were any problematic trees or boundaries and whether materials and machinery could actually reach the building area.

We would also think about the effect on the existing house, how we intended to use the finished rooms and how we would manage while the work was taking place.

Only then would we start becoming attached to exact dimensions or a particular layout.

You don't need every detail resolved before speaking to a professional. The point is to identify the questions that could significantly alter the design while changing it is still relatively easy and inexpensive.

What comes next

This article is only the starting point.

We will look separately at planning permission and permitted development in Wales and England, extension costs and contingency, drains and public sewers, surface water and soakaways, roof structures and joists, structural openings and steels, foundations and trees, boundaries and neighbours, site access and the different professionals who may become involved.

There will also be plenty to cover once the design moves into Building Regulations and construction.

Those subjects overlap, which is exactly what we found when planning our own extension.

Our decision to increase the extension from around 5 metres to 6 metres seemed relatively minor when we made it. It gave us more of the space we wanted, but it also affected things we hadn't initially considered.

That experience probably sums up the biggest lesson from the early stages of our project.

Before changing one part of an extension, it is worth asking:

If we change this, what else does it change?

We wish we had started asking that question earlier.