Before Mark and Angela moved into their new home, they rented privately in Cornwall. In the space of 11 years, they were forced to move nine times. That would be shocking on its own, but when you factor in the couple’s four children, the scale of the disruption starts to look tragic.

They are just one of the families that have been hit by the shortage of homes across England. A shortage which, according to our joint piece of research with Crisis, has now topped four million homes.

In the time-honoured tradition of using British cities or regions as a benchmark for almost any number, that is four houses for every person in Birmingham.

So the question remains – what can we do to address this? Well, on one level, the answer is clear: build more homes. But how many will make a dent, helping to provide homes for people like Mark and Angela?

Most calculations of how many homes we need to build are based on what we expect the population to look in the future. It’s a calculation that weighs up how many ‘extra’ people will need a home that doesn’t already exist.

But if we only look at future demand, we won’t have any impact on this pre-existing backlog: things wouldn’t get any worse, but nor would they actually get any better. We would, in effect, be treading water.

This is why we have also calculated how many homes we would need to build to meet both future need and this existing backlog: around 340,000 homes every year for the next 15 years.

But it’s not just a numbers game – it’s also about the type of homes that we are building. Our figures show that two out of every five should be an affordable home. Still, this covers a huge array of different tenures: from social rent aimed at the most vulnerable tenants, to programmes designed to help young professionals onto the housing ladder.

This is why, for the first time ever, we have also broken down the tenure of the 145,000 affordable homes we need to build every year: 90,000 of them must be for social rent, 30,000 for intermediate affordable rent, and 25,000 for shared ownership.

Of course, getting up to these levels of housebuilding won’t be easy. All told, just 41,000 affordable homes (of all tenures) were built in 2016-17, so there is a long way to go. Housing associations and councils need to work with the government to help boost housebuilding – at the end of the day, we can’t do it on our own, and ministers can’t do it without us.

This is why we are joining with Crisis, Shelter, the Joseph Rowntree Foundation and the Chartered Institute of Housing to call for urgent action from the Government.

Last year, the Prime Minister promised to provide £2bn of funding, which she said could deliver about 25,000 homes for social rent over three years. That’s an average of about 8,000 homes per year – or less than 10 per cent of the amount we now know we need to build. As yet, the government has not released this funding – so we would urge them to do so immediately.

Then, ministers should talk to us about how to support the other 82,000 social rented homes we need each year.

Obviously, funding alone cannot meet the scale of this problem, so we are also calling on the government to make big changes to the way it sells land. At the moment, any surplus public land must be sold to the highest bidder, meaning that housing associations are often outgunned by for-profit private developers. The government should change the rules to make sure that public land is used for public good, prioritising affordable housing development.

Mark and Angela, you’ll be pleased to hear, have moved into a new home built by Coastline Housing down in Cornwall. They pay below-market rent, allowing them to save up so that they can put down a deposit on the house and buy it for themselves. It’s a touching story – they even broke the news to their kids with a poem on Christmas Day – but still far too unusual. As these figures show, there are lots of people out there in the same position. We need to act now to ensure that their story becomes much more common.

James Prestwich is head of policy at the National Housing Federation, which represents housing associations in England, social landlords to 5m people.