It’s not even in consultation stage so it’s difficult to speculate on any proposal at the moment. But the concept has been misrepresented by much of the media.
The idea is partly based on this report, which I recommend reading if you’re interested: https://ukonward.com/reports/a-fairer-property-tax/
See page 14 onwards in particular.
In summary, though, it’s a possible replacement for council tax and stamp duty. It involves an annual levy on all homes to replace council tax (though that will be time consuming and legally complex to implement) and an annual national tax - with payment deferred until the house is sold - on property worth above £500k to replace stamp duty. It essentially shifts the tax currently payable as stamp duty from buyers to sellers. Interestingly, this report proposes not charging it until the next sale of the house - so that if people have recently paid stamp duty they wouldn’t effectively be paying twice.
There would be a lot of detail to discuss and iron out before anything like such a major reform is proposed as policy, so it really is premature even to speculate on how it might be implemented at present. There are issues that I can see (eg what if we start ending up with negative equity again so people are unable to pay when they sell). But it’s an interesting idea at least, and may in some ways end up being fairer than our current systems.
It isn’t simply an additional tax on higher value homes - it’s a potential overhaul of the way we tax property wealth already.