12 Ways to Build the Cheapest House Extension in the UK in 2024

My last post looked into how much it cost to build a house extension in the UK. It turns out there is a lot of demand for this information, so I’ve now made a list of 12 things that will make a house extension as cheap as possible. Be aware, this involves making significant compromises and you might not like it. All design is about compromise. You will have to make a hard choice between keeping costs down or getting some things you really want from your extension.

1. Start at the beginning. 

If you have already begun the design process or actually started building, there is much less scope to reduce costs. You are already committed to the process. You need to decide at the very outset whether this extension is being built to a tight budget. If so, the size, shape and materials in the design need to be chosen for that purpose above everything else. 

It really is vital to declare up front how much you intend to spend. An experienced architect should be able to tell you whether your budget is realistic. Because most of my clients have never built anything before, they don't know what a house extension might cost. So the figure they have in their head might not match the visions they have in their mind. 

2. Do not integrate with the existing house. 

To keep costs to a minimum, the extension should be a box stuck on the side of the house. 

To connect the two, just cut down an existing window or re-use an existing external door. Don't form new openings in the external walls. I've seen time and again how the cost to build an extension shoots up when we start doing work inside the existing house. 

It is possible to use a crude square meter rate to estimate the construction cost for an extension but this does not work inside an existing building. Instead you have to itemise each element of work individually and the older the house, the more likely you will be to encounter unexpected issues.

I realise sticking a box on the side of your home is far from ideal but the point of this post is to show you how to keep costs to a minimum. 

3. Build one storey and no more. 

It you build two storeys the foundations usually needs to be thicker, to cope with the extra load. The new upper rooms have to be integrated with the existing upper floor, and that often means creating new corridors inside the house. It sometimes also means creating new windows for existing rooms. All of this costs far more money. 

There are lots of other reasons not to build two storey, to do with the flow of the house. The best split between public and private spaces, but this video is only looking at costs. So for that reason alone, keep to one storey. 

4. Don’t create new kitchens or bathrooms in the extension. 

The obvious reason why is the cost. Kitchen units and bathroom fittings can be expensive and new drains and plumbing costs money. But the real hidden cost is what you have to do with the original kitchen. You wont have two kitchens in the house, so if you put a new one in the extension, what happens to the original one?

This comes back to my earlier point, do as little as possible inside the existing house. Ideally not so much as a coat of paint. 

5. Don’t do a split level.

This should be self explanatory. And I know, the extension I showed in the last post, about house extensions costs, was split level. But really, make life easy for yourself and keep the new space all on one level. 

6. Use the local building method for the walls and floor. 

Do not introduce unfamiliar materials or methods. You might be tempted to try prefabrication or some new material to save money on your extension but the odds are against this working. 

No matter where you are in the world there will be a standard, local way of building. If you vary from that method, costs go up. Fewer builders will have the skills or experience to use unfamiliar building methods. Mistakes will be made and material will be wasted in the process. Stick to tried and tested, as far as possible, to keep costs down. 

7. Use a flat roof.

Flat roofs get a lot of bad publicity but they use less material than a pitched roof. For extensions, a flat roof will usually have less impact on the exiting roof of the house, thus costing less to build. 

8. Use a suspended floor, if possible. 

Solid floor construction involves concrete slabs, reinforcing, dense phenolic insulation and cement screeds. All of these are expensive.

A suspended floor has a void underneath, it can be insulated using the cheapest mineral wool. And far less concrete is involved. 

It wont always be possible, because there needs to be a minimum drop between the existing house to the external ground. In the UK there needs to be at least 150mm of space below the floor joists to allow ventilation of the area below the insulation in a suspended floor.  So you probably need a height difference of at least 400mm between the existing floor in the house and the external ground to make this work. But is usually costs less to build per square meter compared with a sold floor. 

9. Keep glazing to 1/15 the area of habitable rooms.

The building regulations set a minimum amount of natural light in a habitable room. That’s a bedroom or living space. The minimum in the UK is 1/15th the floor area. So if your bedroom is 15 square meters, you must have at least 1 square meter of glass for natural light. Windows are more expensive than the walls they are built into, so it makes sense to limit their size to the legal minimum.

10. As few lights and sockets as possible. 

Electrical fittings and copper wire don't cost that much but Electricians are probably the most expensive trade in the UK right now. As with the glazing, there are legal minimums for the number of sockets and lights in a room. If you can live with one bulb lighting your room, and as few sockets as possible, you will keep the labour cost for the electrician to a minimum.

11. Fewer trades. 

The UK construction industry is deeply fragmented, with a huge number of small independent tradespeople. It is very unusual today to find a building contractor who has lots of tradespeople on the books, most are subcontractors and that requires lots of management to coordinate. It also increase the chances of mistakes and delays. But you can make life easier by designing out trades. Remove the need for plastering by using facing brick externally. Remove the need for plaster and decoration internally by specifying wall panels. 

Keep in mind what I said earlier about using weird and wonderful methods and materials. Talk to your architect about how to strike a balance.

12. Use a smaller building firm but accept they will take longer. 

Like I said earlier, Managment costs money. Using a smaller building contractor, with lower overheads, will keep costs down. The advantage of bigger, better managed building contractors is that they have more resources and can get thought the work faster. 

So you might save some money but it will take longer. So keep in mind you may need to offset this by paying rent to live elsewhere during construction. 

But that’s a subject for another blog post.

So there you have it, my 12 tips to build the cheapest possible house extension in the UK. Could you live with the compromises necessary to achieve this? Let me know in the comments.  If you have a project in mind and you want to get detailed advice, you can book a consultation with me I will sense check your ideas, timescale, budget and look at issues relevant to the planning and building regulations.