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David Cameron will pledge to turn 'generation rent' into 'generation buy', today, in his keynote speech to Tory Party Conference .

The lynchpin of the speech is expected to be changes to change the definition of 'affordable housing' to include properties worth up to £450,000.

Mr Cameron will argue that so-called 'low cost starter homes' should be included in the definition of 'affordable housing'.

Currently councils can relax planning regulations if developers promise to include a certain ratio of homes to rent at affordable cost - such as social or housing association properties.

The Prime Minister wants to let them include homes for sale to under-40s for up to £250,000 outside London, and £450,000 in the capital in that ratio.

(Image: BBC)

When the policy was first floated , homelessness charity Shelter said it would "primarily help those on very high salaries or couples without children, but they are not a good replacement for other forms of affordable housing and will not help the majority of people on average wages struggling to get an affordable, decent home."

It found working families on average wages would be unable to afford a home in more than half of local authorities .

Today the chief executive of Shelter, Campbell Robb, said: "You don't solve an affordability crisis by getting rid of the few affordable homes we're building, yet that's exactly what this policy will do.

"Today's announcement confirms our fears that Starter Homes costing up to £450,000 will be built at the expense of the genuinely affordable homes this country desperately needs. "

So how much do you need to earn to afford one of David Cameron 's affordable homes?

Mortgage lenders tend to lend up to four-and-a-half times your salary. And that's the maximum you're likely to be accepted for - that figure will go down if your credit score isn't tip-top.

With a little help from Money Saving Expert 's handy mortgage calculator, we worked it out based on the £250,000 cap for England outside London.

Are you...

A single person

£55,500 a year

A home with a single salary would need at least £55,500 to borrow £250,000.

That's more than twice the national average salary of £27,500.

If you're earning George Osborne's National Living Wage, you'd need to work 135 hours a week to afford this kind of mortgage on your own.

Still, at least you'll have a nice place to rest your bones for the six hours a day you're not working.

A couple without children

£33,000 a year each

A bit more manageable, but still requires both of you to earn £5,500 a year more than the national average salary.

If you're both on George Osborne's National Living Wage, you'd both need to work 80 hours a week to afford this kind of mortgage.

And hopefully you'll be lucky enough for your shifts to be at the same time, so you can spend an hour or so in each other's company before you collapse in an exhausted heap.

A couple with children

This is a little harder to calculate - but we've based it on one parent working part time and one working full time.

We've set the part time wage at the national average - which is £112.20 a week - and assumed they'd work 52 weeks a year. Which part time workers rarely do, but we're nothing if not charitable.

So assuming one parent earns £5,834 a year the other would need to earn £61,000 a year.

The main breadwinner, earning George Osborne's National Living Wage, would have to work 149.5 hours a week to afford this mortgage.

That's more than 21.3 hours a day. Let's hope the mother or father of your child is understanding and has stamina.

That's according to the Money Saving Expert mortgage calculator. If we're honest, we're not sure why you need to earn more with a partner than you do as a single person, but apparently you do.