I'm waiting for someone to provide a meaningful counter, rather than snipping out the relevant points and attacking strawmen instead...
If council tax is based on the property, then it has already been paid. Simple really isn't it?
If council tax wasn't a massively flawed system to start with, the debate wouldn't exist.
If you built a fully fledged house at the back, the property would be changed and probably get a new level of council tax to be paid.
Yes, you wouldn't pay more if 5 more people moved into the same house. But council tax is rather based on the sensible occupancy of a house, it doesn't cater for all outliers and the fact that 15 people in a 4 bedroom house would pay less council tax than average per person yet use local facilities.
Its not a perfect system at all, but its not a terrible one either. The one major problem we have in this country is wanting too many services provided by the council but not want to pay for them, council tax is one of the few things I don't have a problem with. We're in debt because we borrow money to pay for services tax can't afford to cover.
Its pretty simple, people are building cheap crap flats out of "sheds" and failing to tell the government they essentially have changed the usage of their land and have an extra income.
Its dodgey, theres no two ways about it, theres a guy living there that essentially no one knows about, the most likely occupants of dodgey back house sheds will be people who don't want to be easily found and can't easily get a flat/house rented themselves for whatever reason.
Don't forget that also neighbours could have paid, I dunno, 500k for a nice house on a quiet street and a nice quiet garden only to have their neighbour install a new tennant at the bottom of the garden who makes their nice garden in their expensive house a smelly noisey tip.
People buy houses knowing whats around them(to the best of their ability) and don't expect to have random shanti town sheds thrown up and occupied.
Its unfair to neighbours, its unfair to those actually paying to live in the area, and depending on the quality/safety/facilities it could be dangerous and hazardous on top.