It is difficult to think of a policy that is as ill-conceived on so many levels as the coalition's announcement on Sunday to allow parents to guarantee their children's mortgages.

Housing is unaffordable today not because buyers are unable to secure yet more credit against the value of their house but because supply is constrained. Not long ago, the average house would have changed hands for three-to-four times average earnings; today, the vast majority of buyers have to pay five-to-seven times average earnings. If you pump more finance into a supply-constrained system, there can be only one result - yet higher prices.

Views differ on the causes of the financial crash and how to deal with the problems that the economy faces today, but one reaction of the government has been to bind banks up in ever-more regulation. Whether that is right or wrong, it is a deliberate policy decision in order to ensure that banks do not fail at the expense of the taxpayer in the future. This has made banks more risk averse. The response by the government has then been to directly take on the risks that the banks have refused, through schemes such as funding for lending or the proposed business bank. This is a bizarre policy. Banks are constrained in their own business models in order to prevent them failing at the expense of the taxpayer and, instead, the taxpayer is now taking on the risks directly.

Clegg's proposal to guarantee mortgages with pensions is another such instance of incoherent policy. In addition to the regulation of bank's capital discouraging banks from risky lending, the FSA is increasingly trying to rein in the provision of mortgage finance at high earnings multiples or high loan-to-value ratios. The government's new proposal seems to work precisely in the opposite direction. Clegg seems to be reasoning that, if everybody can secure their debts on everybody else's assets, then everything will be okay. Is that not the logic that gave us the financial crash in the first place?

Even in terms of the practical details, Clegg's plan seems crazy. Any pensioner who has already reached the age at which they can take their pension is entitled to secure their children's lending on any lump sum they choose to keep as an asset. As such, this proposal is only relevant to future pensioners. If a potential pensioner secures their child's mortgage on a lump sum which legislation prevents them from accessing until at least age 55 what will happen if the child defaults on the mortgage?

Presumably, either the lump sum will have to be taken early - which will cause havoc in terms of the relationship between the lump sum and the rest of the fund which is strictly controlled to prevent tax avoidance - or some complicated contingent loan arrangement will have to be set up. This will all require reams of legislation.

Clegg might also want to ask how many prospective pensioners are so well pensioned that they would be happy to put their pension pot at risk in this way. And, in turn, how many of those prospective pensioners would not, in any case, have a house against which they (or their children) could secure an additional loan for their children if they were so minded?

This is a completely crazy policy which actually works against many of the other things that the government is doing (in some cases probably wrongly) to try to create a more stable financial sector. Parents with assets should have no trouble securing loans for their children if they wish to do so. If banks and parents wish to freely enter an arrangement whereby a pension lump sum is taken into account when negotiating a loan, then so be it - but let's not have the government specially encourage it. The fact that policy proposals in the housing finance area are becoming more and more bizarre ought to focus people's attention on the real problem - the affordability of housing. We cannot make housing more affordable unless supply can respond to demand. Some readers may object to the policy consequences of liberalising development restrictions. However, we should be clear about the housing affordability consequences of not doing so.