We’ve built an online calculator where you can test different council tax changes, so you can see how small changes affect total revenue - and what they mean for individual taxpayers. https://t.co/0V8iWl3FAI

If you want more speculation about speculation, from someone who's clearly no good at speculation, then you can follow me here. Or you can go to the Tax Policy Associates website and sign up for free updates.

(But last year I said the Government would be crazy to increase employer NICs, and I couldn't believe they'd do it. So I'm not very good at the whole "predict what politicians do" thing)

So either: (1) the Government has made a very bad mistake, or (2) they're not thinking of implementing an exit tax. My money is on 2.

The Sunday Times reported that the Government was drawing up plans for an exit tax. If I was a Government thinking about introducing an exit tax, the *last* thing I'd do is give any hint that's what I was about to do. Or people will leave to pre-empt it.

The interactive council tax policy calculator is available on the Tax Policy Associates website, together with a full methodology and link to the code on our GitHub.

Or - even better - a wholesale reform of land taxation, rather than fiddling at the edges.

Instead: I'd add more council tax bands at the top and raise a modest amount of money from people in really valuable properties. And I'd focus on effective ways of taxing wealth - e.g. capital gains tax reforms and more effective IHT reform.

This is all rather unsatisfactory. The only realistic way to raise lots of money from increasing council tax is to hit Band G. These are not the super-wealthy. They've experienced significant tax increases over the previous ten years. I would not do this.

So it's easy to see why the FT says the Government is more drawn towards a simple doubling of rates.

There's also a practical problem: creating a valuation system for applying a percentage tax to the c150,000 properties over £1.5m would require considerable resources (and take time to create). The council tax banding system was designed to avoid such difficulties.

Rates this high would in our view be unwise. They would be capitalised into property values (i.e. because people pay less for property that carries a liability). Meaning a fall in value of 20%+ That reduces SDLT revenue - we estimate by £2bn. It's also unfair.

However a percentage tax fails to raise as much as a simple doubling of Band G and Band H council tax unless we apply rates approaching and exceeding 2%:

We can achieve a fairer result if, instead of a multiplier, we apply a "mansion tax" to all the properties in Band H - i.e. a % of property value within the bands. That has a much reduced impact in those at the bottom of the band and a much higher impact at the top end.

Can't we raise more money by leaving Band G alone, and instead adding new bands at the top? To do this we have to increase council tax bills *five to ten times*. We can't realistically expect people in £1.5m homes to pay a £25k council tax bill. This is not a runner.

That means an average council tax increase of £3,800 for people in Band G. In many cases the owners will earn £100k+ (£67k after tax; more if it's a dual income household). A £4k tax increase is significant for them (particularly for people just in this bracket).

The simplest approach is the one the FT says the Government is considering - doubling council tax for Band G and Band H. Yes, it raises £4bn - but there are so many more Band G properties that they generate 80% of the revenue.

We've built a calculator that lets you model changes in council tax for Band G and Band H. It's on the Tax Policy Associates website (I can't link because X will then hide this post from people; I'll pin the link to my profile. This is really dumb.)

Council tax is based on 1991 property values, divided into eight bands from A to H. Band H is the highest - roughly homes now worth over £1.5 million - and Band G the next, typically £750k – £1.5 million.

The FT reported a "Budget tax raid on the owners of expensive homes", expected to raise around £4bn. But 80% of that comes from the homes in the *second* highest council tax band. As with many tax increases, most of the burden falls on the not-so-rich. Here's why: