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How to build an extension on top of your garage in the UK

Did you know that around 11 million homes in the UK have a garage?

Maybe yours is one of them and you want to learn what it takes to build an extension on top of that garage. I’m not talking about converting the garage itself into habitable space, that’s fairly straightforward. I’m talking about adding another storey on top of the existing garage structure.


I’ve run my own architecture practice in Edinburgh since 2009. I specialise in altering and extending private homes. And for the last two years I’ve helped homeowners right across the UK, by providing advice to anyone thinking of making changes to their property. And this question has come in a surprising number of consultations. 


what to know before extending over a garage
Could an extension be built on top of that garage?

Take this house in England, for example. The client was thinking of buying it but needed another bedroom and they wanted my advice about building an extension on top of the garage to achieve that. On the face of it, this makes sense. I mean, how hard can it be? The garage is right there, just stick another storey on top, extend the corridor through to the new room and Bobs your uncle. 


Not so fast.

it doesn’t matter what kind of garage you have, when it was built or what it’s built from. There are two problems every single-storey structure faces when extending upwards. And I’m going to explain those problems to you and show you how to solve them. I will also show you how I advised this client to get another bedroom without extending over the garage, so stay tuned for that.


Why extending above a garage is not simple

Let’s start with the basics, builders and property developers love efficiency. Getting the most return from the least effort is the name of the game. Why use any more material to build that garage than absolutely necessary? And that’s where the problems start.


Regardless of when it was built or what’s it made from, the existing foundations and walls of your garage are almost certainly not capable of supporting another storey on top. If your garage wall is one brick thick, it’s probably too slender to support the extra weight. And even if your garage has thick, cavity or solid stone walls it is unlikely the existing foundations will cope. 


So how do you solve these problems? Demolishing the garage and building an entirely new structure would defeat the point. Building on top of the garage is supposed to save money. The whole point of the exercise is to get value from adapting the existing structure. 


The first thing to know is that creating new habitable space will require building regulations approval and part of that process will involve a structural engineer. They have to certify that your plan to build above the garage is safe. So the starting point is to work out what’s already been built and what it’s capable of supporting.


Digging a trial hole to see the existing foundation

A trial hole will need to be dug just outside the wall of the garage to expose the existing foundation. The engineer can take measurements, crunch some numbers and then break your heart by confirming everything I just told you. The existing foundation is barely thick enough to cope with the existing garage, never mind adding a storey on top.



How to build above an existing garage

At this stage, there are two possibilities. Let’s suppose the existing walls of your garage are thick enough. Maybe they were built from stone or dense concrete blocks, and they could be built taller, but the foundations aren’t up to the job. 


In that situation, it’s possible to underpin the existing foundation.


This stone wall was underpinned

This is one of my recent projects where we underpinned the existing wall, effectively making its foundations bigger, by digging a hole one meter wide just next to the wall, filling it with concrete, then the next day after the concrete had set, digging another hole next to that, filling it with concrete and so on. 


Underpinning an existing foundation

Its possible to work around the building perimeter, adding more concrete to the existing foundations, bit by bit. I should say this process isn’t always possible. If your garage is built right on the boundary with your neighbour, you can’t have foundations from your structure inside someone else’s land. But this method is viable, provided the existing walls are thick enough to be extended upwards


A normal brick wall garage in the UK

But what if your garage walls are normal, single brick walls. What then. Building on top of this garage involves some structural gymnastics. I’ve done this only once and it was so long ago, I don’t have any photos to show you, so Ive made this 3D model instead. 


The existing garage structure

The idea is easy to understand. We want to leave the existing foundations and walls alone and build a completely separate structure inside the garage to support the new extension above. 


New pad foundations shown in red

First off, we dig four large holes, one in each corner inside the garage. Into these, we pour lots of concrete. These are pad foundations and onto each one we bolt a steel post, as close to the wall as possible. 


Steel posts in each corner of the garage

These posts extend up to the existing roof, which will be stripped off because we don’t need it anymore, and a series of steel beams join the posts together. These beams are collectively known as a ring beam.


A steel 'ring beam' to support a new extension above the garage

The ring beam forms a new deck, or floor, onto which we build the new extension above your garage, leaving the existing structure largely untouched. You could convert the existing garage into habitable space, line and insulate the inner surface of the garage walls and hide those new steel posts within that insulation, its all possible.


It is worth extending above a garage in the UK?

And at this point, we should discuss cost, or at least value for money. Is it worth doing this? The honest answer is, it depends. 


The amount of underpinning work or new steel frames required will vary massively, depending on the existing structure. It's not possible to give you a ballpark figure but even for a single-car garage I would expect many tens of thousands of pounds. And to extend above a double garage could end up costing a hundred thousand, or more. 


The value of that new space will vary depending on how much one more habitable space is worth in your part of the country. This is what I told the client who booked a consultation to get my advice about this property in England. 


The existing ground floor layout

How this house could be re-designed without extending above the garage?

The client for this property had a budget of £150k, and wanted to refurbish the interior, create a better kitchen, and dining space and add one more bedroom. 


I suggested that, instead of building above the existing garage, they convert the garage and join it into a new open plan kitchen, living dining space. That would then allow the existing sitting room and study to become a master bedroom and ensuite bathroom, respectively. 

How the layout could be changed

The upper floor could be re-arranged to make the layout more efficient. All of this could be achieved within their £150k budget, assuming new kitchen units aren’t expensive and there’s nothing fundamentally wrong with the fabric of the existing house, like rot, subsidence or a defective roof.


Summary

So if you are thinking about altering or extending a home anywhere in the UK or buying a house that needs to be adapted to suit your needs, check out the Real Life Architecture website. It's full of free guidance and blog posts packed with useful information for homeowners. 


You can also book a consultation with me if you need specific advice about your property. My name I Níall and I hope you found this video useful. 

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