They wont but it doesn't matter if it makes a good soundbite which is how all politics is conducted these days. See also their plan to hypothecate higher rate tax for more nurses as if money alone will magic up a load of willing people.

What Chuffy said, its a pie in the sky political soundbite.

Labour isn't going to build the houses, is it?

Either local authorities will - they could change the rule preventing them from spending right-to-buy income on building more homes - or developers will. That may require financial incentives, probably, but that will merely be a reduction of income rather than an expenditure.

Remove all second home ownership tax benefits. Take CGT on second homes on an annual valuation estimate balanced on sale. Crush small time BTL and encourage rental REITs with secure tenancies and set profit margins over financing costs (i.e. treat housing like a utility). Build houses out of SIPs in a fraction of the time it takes to build out of block/brick. 

Impose use it or lose it on all major builders' land banks or simply appropriate them and get fast track planning through. Impose CGT on primary residences but give a one off exemption for over 60s downsizing. 

I'm assuming the private sector will build them but that still doesn't get you past the issues around planning, labour supply, financing and infrastructure.  Yes it's lovely that the developer has to build a new GP surgery and a bus stop but that doesn't help with the increased traffic through the centre of the village and no developer can afford to build a three mile bypass off the back of building 100 new homes.

They've never done anything about logistics though have they? My parents used to live on a mega estate in Kent where they build a new road down to the still tiny station with a too small car park and the same number and size of trains to London. Presumably the answer in the south east, apart from freeing up underused stock or banked land, is to have new towns built on crossrail 2. 

In the end it is taking land off people at a below market price and saying fook you to neighbours so as stated above will likely never happen.

Stop subsidising people to work in London (and other big cities).  Make employers bear the full burden including the cost of housing, schools, hospitals and transport for their employees.

Move the work to the 300,000 empty houses.

Obviously some jobs like nurses, firefighters and binmen do have to be on the ground, but there is no reason why taxpayers should be contributing towards keeping employees pushing paper and tapping on screens in expensive offices with expensive burdens on local public services.