By now, many of you will have heard, or at least heard of, LBC radio’s interview with Green Party of England and Wales (GPEW) leader Natalie Bennett in which she stumbled badly over the detail of her (excellent) policy to build 500,000 new council houses over 5 years.

As Guardian writer Zoe Williams said today, the impact is vastly overstated by people for whom politics is their favourite spectator sport. In reality, there are far, far worse crimes than blowing an interview, and the leaders of the establishment parties have treated us to a devastating selection of them over the years. The fact that more people joined the Green Party yesterday than on any other day over the past fortnight suggests that actually, many people would rather have a politician who tries to make a difference even if it’s hard, than explain with ease why nothing can change.

But one of the most frustrating outcomes of yesterday was that the impression was given that because Natalie didn’t have the figures to hand, they didn’t exist. They do. So I wanted to set out exactly what the answers to Nick Ferrari’s questions actually are, for anyone who is wondering.

The GPEW proposal is to build 500,000 new homes by 2020. Good-quality, sustainable social housing can be built for around £100,000 per unit in upfront costs; but for medium to long-term budgeting purposes you should then factor in the revenue stream the house will generate, paying back on its own build cost. So GPEW estimate an overall cost of £60,000 per house, for a total of £30bn over the 5 years.

Of the £30bn, £7.5bn, £1.5bn per year is already in the budget. The bulk of the remaining £22.5bn will be raised by abolishing the subsidy to buy-to-let landlords which allows them to pay no tax on their mortgage interest payments. This would raise at leastGPEW’s very conservative estimate of £4.5bn per year by 2017, but since recent reports value the landlord subsidy at £5.8bn and beyond, it’s actually very likely it could pay for the whole project on its own. You can see all these figures in the table below:

If, after abolishing the landlord subsidy, there is still some money left to find, it will be covered from the savings that will automatically arise from the programme itself: reduced housing benefit payments, reduced costs of healthcare and emergency accommodation, and increased tax revenue from the economic stimulus of all that construction work. For every £1 spent by the public sector on housing, 56p returns to the Exchequer in tax revenues and benefit savings (see page 11 of this report for the UK Contractors Group).

You can read all of these details and more in the Green Party of England and Wales policy brief, 500,000 Social Rented Homes by 2020.

So there’s the missing detail, fully costed and fully funded. Maybe that’s what Natalie should have said. But then again maybe not, or at least not all she should have said. Because while we need to be able to answer questions of detail, our purpose is to change a political and economic system that everyone can see is failing, not just explain how our plans are compatible with it.

I think it’s high time that people who want to demand explanations of how we can afford to meet the basic duty of putting a roof over the heads of every person and family in this country are first asked: how can we afford not to?

Gary Dunion is Communications Co-ordinator of the Scottish Greens and a potential Scottish Parliament candidate for Central Scotland. This piece was originally published on his blog here.