It depends enormously on what's already there, more than most jobs.
At the light end, a house that's already got the right layout and enough bathrooms might just need bringing up to standard. Fire doors, a linked alarm, emergency lighting, some fire stopping and an EICR. That's usually £10,000 to £20,000 on a typical terrace and you're not touching the fabric of the building.
At the other end, if you're adding bathrooms, moving walls, taking the reception room into a bedroom and doing the loft, you're into a proper conversion. That runs around £1,150 to £1,900 per square metre managing it yourself, or £1,440 to £2,375 if a builder runs it. On a 90 square metre terrace going to five or six rooms, expect £100,000 to £170,000.
Most jobs land somewhere between the two, and where yours sits depends almost entirely on how much you're changing rather than how big the house is.
The costs that push it up are mostly the ones you can't see afterwards.
Fire safety is the big one. Fire doors throughout, usually FD30 with self closers, a linked alarm system, emergency lighting on the escape route, and fire stopping between rooms and floors. On a six bed that's easily £8,000 to £15,000 and it's not optional.
Bathrooms are the next. Most councils want one bathroom per three or four tenants, so a five bed usually needs two. Each one is £6,000 to £10,000 once you've allowed for the drainage, and the drainage is what catches people out, because a new bathroom on the wrong side of the house can mean lifting floors or running a macerator you didn't budget for.
The kitchen has to be sized for the number of tenants, which usually means more units, more worktop, sometimes two cookers and two fridges. Budget £8,000 to £15,000.
Then there's the electrics. Most conversions end up with a full rewire because you're adding circuits, and you'll need an EICR afterwards. £4,000 to £7,000 on a typical terrace. Heating often needs upgrading too, since a combi that was fine for a family won't keep up with six people all showering in the morning.
Room sizes are set by licensing law in England and they're worth checking before you draw anything. A room for one adult has to be at least 6.51 square metres, and 10.22 for two sharing. Anything under 4.64 square metres can't be used as sleeping accommodation at all. There's no point planning a six bed if one of the rooms doesn't qualify.
Councils add their own standards on top, and they vary. Some want a minimum communal living space, some have parking or bin storage requirements, and some won't accept a bedroom without a window of a certain size. Read the local standards before you commit rather than after.
The thing that kills more HMO schemes than cost is planning. If the property is in an Article 4 area you'll need planning permission to convert, and a lot of those councils also cap how many HMOs can sit within a certain distance. Worth checking that before you price anything.
Licensing is separate again. Five or more tenants from more than one household needs a mandatory licence, and plenty of councils licence smaller ones too. Expect £500 to £1,500 for five years depending on the council.
One last thing. The finished value of an HMO isn't usually what a comparable family house sells for. Smaller HMOs often value on bricks and mortar, which means you might spend £120,000 and add considerably less than that to the value. Larger ones, typically seven beds and up, are more likely to be valued on the income they produce, which is where the money is. Worth knowing which one you're building before you start, because it changes the whole case.
Avalor prices this using BCIS benchmark rates, adjusted for your postcode and set to self delivery, so the figures assume you're running the job with your own trades rather than handing it to a builder. If you've got a real quote you can put that in instead. It also checks Article 4 and local planning history as part of the appraisal.