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How to save money on a house extension in the UK

Updated: Apr 7

How to build the most cost efficient house extension possible in the UK
A house extension I designed and built in Edinburgh, 2015

Like all construction projects, house extensions have become increasingly expensive over the last few years. Since Covid, inflation in the UK building industry has been running rampant. It’s become so bad that I’ve given up advising clients on what their projects are likely to cost. While most of my projects are high-end and have bigger than average budgets. That does not mean my clients have infinite money. They have to draw a line somewhere.

So how do you save money on a house extension in the UK.

I’ve split this post into two parts;

The first looks at big strategic decisions such as the size and layout of the extension.

The second part of the video looks at materials and construction methods.

Keep in mind that design is always about compromise. Whether it’s clothes, cars, electronics or buildings, we always face a choice between time, cost and quality. If you want to build a house extension for the lowest possible price, then you have to accept serous compromise in time and quality. You may think my suggestions are extreme but they should help you understand how to go about designing the maximum house extension for minimum budget.

First off, the easiest way to build a house extension is not to do any alteration to the existing house. If you want the cheapest possible extension just stick a box on the side of your home and not so much as a coat of paint inside the existing building. Where extensions go wrong financially it’s when they start messing with the layout of the existing building.

I should say this is not something I advise my clients to do. Most of the properties I deal with involve significant internal alterations as well as house extensions. Maybe the property is old, maybe it needs substantial refurbishment, maybe the kitchens, bathrooms and bedrooms are in the wrong place and so we have to change the layout of the building. But if we’re looking at this purely from a financial point of view, the cheapest way to build a house extension is to just stick a box on the side of the house and not worry about the existing building. The downside of this is that the house and the extension will operate like two different structures. If you want a home that flows properly you have to integrate the extension into the layout of the house, there’s no other way of doing it but that comes with a financial penalty. If your budget is limited, it will mean you get a smaller extension because a huge chunk of your budget is being spent inside the existing building.

The second way to avoid spending more than necessary on a house extension is to keep it one story. I have lost count of the number of people who phoned me up saying the must have a two story house extension. Most of the time they’re just thinking in a linear way. Their existing house is two stories with the bedrooms on the upper floor and a living space on the ground floor. If they want more bedrooms it follows that they have to extend on the upper floor, right? Wrong!

This creates significant problems, particularly when you have to integrate the new bedroom into the upper floor of the house, it doesn’t always work out smoothly. Maybe the stairs is in the wrong place and a new corridor must be built on the top floor. That corridor will steal space from existing bedrooms. Maybe the amount of new space required on the upper floor doesn’t match the new space on the ground floor, and the new structure will be awkward to build. Maybe altering the exiting roof and external walls in order to integrate the new upper floor extension will be prohibitively expensive. And all of this before we consider the implications for a planning application. A larger structure can overlook and overshadow your neighbours, usually a major problem for the planners.

For those reasons, I almost always advise people against building two stories. A better way of doing things, and I know this advice contradicts the first point I made, is to rearrange how the house and the extension work as one unit, rather than two separate things.

Any well designed home must have a clear split between public and private spaces. Public spaces, like your kitchen or living room, is where you feel comfortable entertaining guests. Private spaces are the bedrooms, bathrooms or your home office. So if you have an existing two storey house, and you need more bedrooms, it often make sense to create a larger kitchen / living / dining space on the ground floor by incorporating part of the existing house into the extension. Like in this project.

photo of house extension during construction, showing corner of house demolished
The existing corner of the house was taken down, and the room incorporated into the new extension

You can then create a new main entrance to you home, which walks straight into the new space. The advantage of this approach, is that existing rooms on the ground floor which were used for dining or living spaces, can now become additional bedrooms. In effect, the existing house becomes a dormitory and the new extension become the public or entertaining space. Looking at it like this is the most effective way to extend a house but its not the cheapest. Keep in mind that if you decide to build a kitchen or bathroom in a room which isn't already a kitchen or bathroom, it will cost more. Because forming new drainage can be expensive.

interior of house extension with open plan kitchen, living, dining space
This is the finished house extension, shot from the same vantage point and the previous image

Another design decision that can keep the costs down is to use a flat roof, rather than a more conventional pitched roof. There are two reasons for this. First, there is less material in a flat roof than a pitched roof, covering the same area. So it costs less to build. Second, a pitched roof usually has to be integrated with the exiting roof of the building and this takes time. A flat roof is easier to connect to the existing building without having to take apart the exiting roof to do it.

Finally, if you want to design a house extension for a minimum budget, keep the number of corners and junctions to a minimum. A square or rectangular layout is going to be cheaper to build that a complex layout. And absolutely no curves or weird angles. Those cost a fortune to build.

Now that we’ve discussed the big strategic decisions that can either cost or save you money, lets talk about the material choices that affect the budget. No matter where you are in the world there will be a typical or standard method of construction that local builders prefer to use. In Scotland, where I live, its timber frame construction. The point is, if you chose to vary from the preferred method of construction it will either cost far more than normal or you wont be able to find a contractor willing to build your project. Take advice from an experienced local architect on this one. In the UK, most homes are built with masonry on the outer walls. That is brick, block or stone. Sometimes these have render of plaster over them but the basic construction is the same.

While this is traditional, is it time intensive and cheaper alternatives are available. The most cost effective external wall materials is cladding. Whether that is timber cladding, metal cladding like zinc or pre-formed cladding panels. An external wall with cladding will be thinner than a wall with masonry, so the foundation underneath that wall can also be thinner. Saving more money. Im not going to get into the issues of which cladding material is best or whether any cladding is acceptable to planners or building regulations. As always, take advice from a local architect.

Another way to save money is to reduce the amount of glazing in your extension. Per square meter, windows usually cost more than the wall they are built into. Keep in mind that in the UK, the building regulations set a minimum amount of natural light for habitable rooms, so you cant eliminate windows entirely from bedrooms or living areas. I should also point out that the cheapest material for windows is UPVC. Timber and aluminium cost way more.

Next up is the ground floor construction method. These usually come in two flavours. Solid concrete floors and suspended timber floors. A solid floor requires a lot of digging and concrete, as well as more expensive, dense insulation. A suspended timers floor has a void underneath and the insulation can be mineral wool, which costs less. The downside of a suspended timber floor is that it has to be raised 200 - 300mm above the external ground, because the void underneath must be ventilated. Not everyone is happy with suspended floors, some people notice that they move when walked on, while solid concrete ones don't. Its also much easier to fit underfloor heating into a solid floor.

Now, if you really want to push cost efficiency to its extreme conclusion, take a drive to your nearest out of town retail park or industrial estate.

Photo of retail park
Typical out-of-town retail park

These big boxes are the cheapest way to enclose space. They use steel frame and profiled metal cladding on the walls and roof. Im not saying your extension should resemble a shrunk down Ikea but if you want the most space for the least money, a steel frame with profiled metal cladding is the way to do it.

steel frame and cladding structure
Would you build a house extension like this?

While it would raise all sorts of questions from a planning permission and building regulations standpoint, I suspect it is feasible, the question, is would you want to?

I should also point out that pre-fabricated buildings or kit homes are not the answer. This might work if you are a property developer building dozens of houses in a field but its not going to save you any money if you are extending your home.

If you are planning to build a house extension anywhere in the UK, and you would like advice from an experienced architect, get in touch and book a consultation with me to discuss your ideas, timescale and budget. I provide a sense check on your ideas and can advise you on how to find the right local architect for your project.


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