Side Extension Design: The real question isn’t “can I build it” — it’s “what actually fits”
Clients often start a side extension conversation with an outcome in mind — usually a bedroom, a playroom, or an extra living room — without accounting for how much usable internal width they’ll actually end up with. The starting external footprint and the finished internal room size can be surprisingly different once you account for wall build-up. In practice, a lot of these narrower spaces end up being used as something else entirely: a home office, a utility room, or a downstairs WC. That’s not a compromise so much as a better match for the width that’s actually available.
Where the width actually goes
A lot of side extensions replace an existing garage, and garages are often built from single-skin brick, single block, or even prefabricated concrete post-and-panel construction — thin by modern standards. Once you extend it properly, you’re building in a cavity wall, which is significantly thicker.
Here’s a realistic example: a garage that’s 2.4 metres wide externally might only be 2.3 metres internally. Once you rebuild that wall as a proper cavity wall — going from roughly 80mm thick to something more like 300–350mm — you can lose another 200mm of internal width. Then there’s plastering the original outside wall of the house it’s joining onto, which can cost you a little more depending on whether it’s dot-and-dab or boarded to run cables behind. By the time all of that’s accounted for, a garage that looked like 2.4 metres of space can end up closer to 2 metres internally.
That matters because a single bed alone takes up close to a metre of width. A 2-metre-wide room doesn’t leave much to work with for a bedroom.
What clients usually ask for, and what actually works
Bedroom, playroom, and extra living room are the three most common starting requests for this kind of side extension. At the widths these spaces typically end up with — around 2.4 to 2.5 metres once wall build-up is accounted for — none of those three are a comfortable fit. A single bed alone takes up close to a metre of width, and a playroom or living room needs enough clear floor space to actually be usable, not just technically fit.
What does work well at that width:
– A downstairs utility room (only needs around 1.8 metres)
– A downstairs WC or shower room
– A home office
– A boot room
This is why so many of these rooms end up as an office, a utility room, or a WC rather than the bedroom or playroom the client originally asked for — not because the build was scaled back, but because those are the uses that actually suit a narrower footprint well.
The better option for a lot of semis: extend the kitchen instead
Many three-bed semis have the kitchen in the back corner of the house, at around 2.5 metres wide — narrow by modern standards. If you’re extending the full depth of the house anyway, taking out the side wall of the existing kitchen and extending into the side extension can nearly double that width.
That opens up options that a standalone bedroom side extension doesn’t: a proper kitchen-diner, a small utility tucked into the darker middle section (sometimes alongside a downstairs toilet), and a study at the front where the light is better. For a lot of clients who start out asking for “a bedroom,” this ends up being the more useful outcome once they see what the numbers actually allow.
What this means for you
Before settling on what a side extension will become, get the real internal width worked out first — garage width minus cavity wall build-up minus finishes, not the external footprint. If the number that comes out is under about 2.4 metres, a bedroom is a tough ask, but a utility room, shower room, office, or an extended kitchen layout usually isn’t.
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