It depends far more on what you're actually changing than on the size of the house. A lot of the building usually stays as it is, so pricing it per square metre overstates the job unless you're stripping the whole place back.
A straightforward two flat conversion, where the layout broadly works and you're not doing structural work, tends to land somewhere between £60,000 and £120,000. That covers separating the services, the sound and fire separation between the units, a new kitchen and bathroom for the second flat, sorting the common hallway and making good.
You only get up towards £200,000 and beyond if you're also rewiring and replumbing throughout, converting the loft, moving walls around or the building needs proper structural work. At that point you're effectively doing a full refurbishment as well as a conversion, and the per square metre rates for a full refurb are the better guide.
The costs that push it up aren't usually the ones people expect.
Separating the services is the big one. Each flat needs its own consumer unit, its own heating, its own hot water and ideally its own gas, electric and water meters. That means new supplies, and if the meter position is wrong or the incoming supply isn't big enough, the utility company charges you to move or upgrade it. Allow £8,000 to £15,000 per flat, more if anything has to come in from the street.
Sound separation between the flats is a building regs requirement and it isn't cheap. Floors usually need building up with resilient layers and an independent ceiling below, and party walls between flats need treating too. It also eats ceiling height, which matters if you're already tight. £5,000 to £12,000 depending on the area involved.
Fire separation is the other regs item. Each flat has to be its own fire compartment with 60 minute separation, fire doors onto a protected common hallway and usually a linked alarm system. If the only escape route is through the shared hall, that hall has to be properly protected. £6,000 to £12,000 on a typical house.
Kitchens and bathrooms are the next chunk. A new kitchen runs £8,000 to £15,000 and a bathroom £6,000 to £10,000. The drainage is what catches people out. A new bathroom on the wrong side of the building can mean lifting floors, running a new soil stack or fitting a macerator you hadn't budgeted for, and an external stack sometimes needs planning consent of its own.
Unlike most loft conversions, converting a house into flats always needs planning permission. It's a change of use and it isn't permitted development. Councils vary a lot on how they treat it. Some are supportive where there's demand for smaller units, others resist it in areas they think are losing family housing. Parking is often the sticking point, especially in outer London and the suburbs where policy expects a space per unit.
Worth checking whether the street already has a run of converted houses. If it does, there's precedent. If yours would be the first, that's a harder application.
The legal side catches people out too. You'll be creating leases, which usually means keeping the freehold and granting long leases on each flat. That's a solicitor's job and it costs more than a standard sale. If the building is already leasehold, you'll need the freeholder's consent before you do anything, and they can charge for it or refuse.
On value, two flats are usually worth more than the house was, which is the whole point. But not always. In areas where family houses are scarce and flats are plentiful, the gap can be small enough that the work isn't worth doing. Check what two flats of that size actually sell for on that street before you commit, rather than what the average flat sells for across the district.
Avalor prices conversions using BCIS benchmark rates, adjusted for your postcode and set to self delivery rather than a builder running the job. It also checks the local planning history so you can see how similar applications nearby have gone. If you've got a real quote, you can put that in instead.