Those wondering what David Laws is up to or lamenting his departure from politics can relax – or not, depending on their point of view. The ex-chief secretary to the Treasury is at work inside Liberal Democrat HQ in Cowley Street on a document called Tax 2020.

Laws, alongside his successor as chief secretary to the Treasury, Danny Alexander, and the party's economists have been working on a secret project since the beginning of the year. Their mission is to detail where they want the party to be on tax in 2020 and then work back from there to see where they should be at each stage – now, in next year's budget, then the penultimate budget before the 2015 election and, of course, in the next parliament – be it in coalition with the Tories or maybe even Labour.

They are not alone in tax planning. Before Ed Miliband offered the other Ed the role of shadow chancellor, Balls told him they would need to come up with some tax cuts, because the other parties would. He is clearly in a hurry – his call for an unfunded temporary VAT cut may have been sprung on shadow cabinet colleagues because he wants Labour to get into this territory fast. Balls agrees with Laws who agrees with Osborne: 2015 will be about tax cuts.

The Lib Dem document is more chunky than a hasty unfunded tax pledge, or anything we know their Tory colleagues to be up to. Oliver Letwin, the Cabinet Office minister and Tory policy guru, has told his backbenchers to start doing some intellectual heavy lifting because he and the front bench are kind of busy.

For sheer brazen go-it-aloneness, the Lib Dems' work on tax is rivalled only by the home secretary Theresa May's planning for a purely Tory policy on reforming the relationship with Europe's courts.

The section I have seen suggests the Lib Dems are close to sorting out their problems with their mansion tax. This policy had become an embarrassment, crumbling like a stinking chunk of policy cheese in Vince Cable's hands. One Clegg aide, also now a member of Tax 2020, expressed relief on entering government that the coalition with the Tories meant they would not have to implement this complicated tax.

But over time they have returned to it because it helps to highlight their distinctiveness. While, as they see it, Tories and Labour simply tinker with the tax system, they want to do a major overhaul. It should be about taxing unearned wealth (namely, property) rather than earned income. Tories and Labour, they think, are hung up on income. The influence of their lodestar John Stuart Mill is felt here. He saw income tax as a tax on effort and ability.

A clear emphasis on tax reform is something that singles the Lib Dems out from their rivals, thinks Gavin Kelly of the Resolution foundation who when in Number 10 practically invented the term the "squeezed middle".

So, scrapping the 50p rate of tax? They'll probably acquiesce when Osborne does it, but according to one Lib Dem close to Clegg: "It's a sideshow." However, when Cameron brings forward his "marriage tax", Lib Dems say they simply will not vote it through parliament.

Their new policy sees the mansion tax modified to a mansion sales tax: people will be charged a higher rate of capital gains tax on any profit made when selling a million-pound-plus house. The CGT would be levied at least at 28% on that bit of the profit above £1m. Their mansion tax policy had eventually proposed an annual tax of 1% on £2m properties, so in this sense, their new policy could affect more people.

To make this less onerous they want to encourage people to renovate a house rather than move on, by reducing the VAT on improvements to 5%. And they think they should scrap stamp duty for low earners. This, and other items within the document, will be challenging for George Osborne. Many economic liberals in the Tory party would prefer a tax on property wealth at the point of sale to the 50p rate of tax for instance but it goes against their political interests as it would be unpopular with a lot of their voters.

It is well known that the preferred Lib Dem way to offer a tax cut is through another increase in personal tax allowance. JS Mill supported a large tax-free allowance sufficient for "life, health, immunity from bodily pain" and it is this policy and their very effective communication of it that sees the Lib Dems poll so well on tax.

The two parties agreed that taking people out of tax is a good thing – it was described as "the glue that binds" the coalition. But they may be beginning to veer away from each other. Osborne is pressuring Boris Johnson to stop London Underground members of staff getting free travel. Tories believe that until underground managers and executives pay for their travel they won't keep fares down because they don't need to.

And one cabinet minister says: "I fundamentally dislike their income tax policy. It is the polar opposite of no taxation without representation – the less well-off are represented, just not taxed. It means you're removed from society. You are not invested in society."

This is a minority position, and it is hard to see Osborne opposing anything the Lib Dems do in this area apart from for one reason. As one observer said: "Osborne didn't come into politics to be Nick Clegg Plus." He may want to come up with his own tax cut. But anything he does has to rival the Lib Dems for simplicity.