HMO statutory room size discretion?

I think it is very unlikely any bedroom under the statutory minimum size is going to be acceptable unless there is other space given to the occupier of that room for their sole use as a compensating factor.

As I understand it the 6.51m min is a countrywide minimum that there is no discretion on. We had two properties with a room smaller than this with a very happy tenant in each, but sadly rather than leave an empty room we had to sell both properties and evict the 8 happy tenants, which was bad indeed for them and us... a very bad regulation which hits the people at the bottom hardest. (I understand that the new landlord is letting this room, and indeed has divided the largest room into two as well, which is good for homelessness and the tenants, but is, pathetically, illegal.

The 6.51m2 is already discretionary. The national minimum is 8m2 for a single occupancy room but this can be reduced to 6.51m2 at the discretion of the Licensing Officer if there is sufficient communal amenity.
If it is a HMO License you are applying for they will most likely issue a ‘temporary prohibition order’ on this room requiring you to either increase its floor area to 6.51m2 or cease letting it within 18 months.
Furthermore any mortgage valuer noticing a room under 6.51m2 will most likely not take into account the rent received from that room when calculating gross rent.
We have had to demolish and move walls in many houses, even when rooms were 6.48m2!
Sorry to be the bearer of bad news but the only solution is to either enlarge or not rent that room

Two or more room units refers to where 2 or more rooms within a house are let to one tenant (or a cohabitating couple), not where there are just 2 or more rooms in the property let. A ‘unit of accommodation’ is a single room where that is the part of the property that the tenant has exclusive use of.
In any event the minimum acceptable bed room size won’t be under 6.5m2

I expect that, most likely and unsurprisingly, the Council's website is out of date because you really can't expect Council staff to ensure that their website reflects the true, current legal position! Before the law changed and these minimum room sizes became mandatory, our Council did exercise some discretion, to their credit. When the law changed, we had to move the stairs around where they turned onto a half landing (that took a bit of thinking out) to gain the necessary space for a room over a kitchen in a typical Victorian terrace at a cost of around £2,500. This was much better than having to sell the house or lose the rent from that room though.

If the room has a build in wardrobe, the removal of the doors and hanger rails and increasing the internal height to ceiling level will mean it now forms part of the room and can be included in the measurement, there is nothing stopping you from then inserting a standalone wardrobe into the same space! The size is no longer discretionary as it is now law. LA's can, however, prescribe a bigger minimum size room as part of licensing criteria.