Is This Property Big Enough for a Large HMO? And How To Work This Out…
( 4 Min Read )
One of the most common questions I get from investors who are serious about Sui Generis HMOs is a deceptively simple one.
What size of property do I actually need to buy?
Not which street, not which town — but what size, what configuration, and how do I know whether a building I'm looking at can deliver the number of bedrooms I need?
Many investors guess. They look at a four or five bedroom house, think it feels big enough, and factor an 7 or 8-bed HMO into their numbers without ever properly checking whether the building can actually deliver it. Sometimes they're right. Often they're not — and by the time they find out, they've already offered.
There is a better way.
How The Floor Area Actually Breaks Down in a Large HMO
Every square metre of a large HMO serves one of three functions: bedrooms, communal space (kitchen / living room), or circulation (hallways, landings, stairs, laundry rooms or utilities / service areas).
Understanding how floor area typically splits across these three categories — and what that means for the total size of building you need — is the starting point for any serious property assessment.
I went back through five of our recent large HMO projects (8 to 12 bed HMOs) and analysed exactly how the floor area broke down across each one in percentage terms. The results were consistent enough to produce a reliable rule of thumb that any investor can use at the assessment stage — and it's something I share directly with clients when we work together on a project or during a 1-1 power hour (online 60 MS Teams meeting).
Working Backwards from Your HMO Bedroom Target
Once you know how floor area typically divides across a large HMO, you can work backwards from the number of bedrooms you're targeting to the total GIA (gross internal area) you need to find.
When it comes to bedroom sizes, I work to a minimum of 8m² — but ideally I'm designing bedrooms at 10–12sqm+. Each en-suite adds approximately 2.5-3 sqm on top of that. So each bedroom unit (room plus en-suite) sits somewhere between 12.5 to 15 sqm at a minimum.
That gives you a working range for each bedroom. Multiply that by your target bedroom count, apply the typical floor area split, and you arrive at a total GIA target — and that's the number to take to your property search.
The other important point: that total GIA doesn't all have to exist today.
Extensions and loft conversions count. A property that's currently 130m² (for example) might still work for an 8-bed HMO scheme if it has the footprint to extend and a loft worth converting.
The key is assessing the potential total, not just what's standing in front of you.
But Floor Area Isn't the Only Constraint
This is the point that catches even experienced investors out.
Physical space is necessary — but it isn't sufficient on its own. The other major constraint is windows.
Every habitable room in a large HMO needs a window that provides adequate natural light and a reasonable outlook. A window that faces directly into a narrow gap between buildings, or looks out onto a blank wall two metres away, is unlikely to satisfy planning officers — and will affect the quality and rentability of the finished room even if passed at Planning.
This matters most with deep plan buildings — properties that are narrow at the front but extend a long way back. The further back you go, the harder it becomes to get meaningful natural light into rooms.
A building that appears to have the floor area for ten bedrooms might realistically only have good windows to serve seven bedrooms.
So at the initial assessment stage, always run two checks:
Does the total floor area — existing plus any realistic extension or loft potential — get you to the GIA you need?
Do you have enough viable windows to serve that number of bedrooms and communal spaces?
Both need to stack up.
If the window count falls short of your bedroom target even when the floor area works, you need to either rethink the bedroom count or find a design solution — and that's a conversation worth having before you make an offer, not after.
The Quick Sense Check
If you want a back-of-envelope check on any property you're considering for a large HMO:
Estimate the total GIA you could achieve, including any realistic extension or loft potential.
Apply the typical bedroom area percentage to get your approximate bedroom space.
Divide by your target bedroom size (including en-suite) to get your approximate bedroom count.
Then look at the windows and ask honestly whether the number stacks up.
It's not a substitute for a proper design feasibility assessment — but it will tell you very quickly whether a property is worth spending more time on.
Want to know the exact floor area percentages from our five completed projects — and how to apply them to a property you're looking at?
Book a 1-1 online power hour call and we can talk it through so that you are crystal clear on the size of the property you are looking for and even live review some potential deals with you.