If my area is anything to go by, they could probably gain more by rebanding all the properties that have been massively extended. The houses here and in the adjoining 3 streets were originally built as either 2 beds/2 recs/upstairs bathroom (c/tax band C) or 3 beds/1 rec/ground floor bathroom (band D). The footprints are identical, regardless of number of bedrooms.
Most of the houses have now been extended, some several times, but the council tax bands are unchanged. Some of them are now more than twice the original size.
Over the years, a former 2-bed has had a full-depth, two-storey side extension, a full width single-storey rear extension, and a loft conversion. It is now a massive 5-bed, 3-bath, 3 reception room house but it's still banded C for council tax. It's worth approximately 75% more than it would have been without all the extensions.
There are 5 adults living there (parents and grown up kids) and they only pay 25% more in c/tax than I do, living alone in a house less than half the size.
And they piss off the whole street, because they have 6 vehicles between them, all but one of which are parked on the road, because they no longer have a garage and a drive!
And then there's the family that have had a self-contained "garden building" erected, which they let through airbnb or similar. It seems to have a very high occupancy rate, there are people staying there most weekends and often for a night or two in the week. The house is still banded C, despite having what is effectively a studio flat in the garden.