The London mayor, Sadiq Khan, has outlined plans to quadruple the proportion of “affordable” housing being built in the capital, warning he will target “greedy developers wishing to get maximum bang for their buck”.

Khan criticised his predecessor Boris Johnson for leaving “the cupboard bare” and said just 13% of new homes in the planning pipeline are currently classed as affordable.

At the start of his second week in office, the Labour mayor told the Guardian he wanted more than 50% of homes on some new housing developments to be affordable. He said that did not mean 80% of market rent, as affordable is defined by the government, but far lower social rents or “London living rent”, which is pitched at a third of average incomes.

Khan also announced he was considering making it a condition of planning permission that new homes were marketed locally for at least six months before they could go on sale to foreign investors.

“There is no point in building homes if they are bought by investors in the Middle East and Asia,” he said. “I don’t want homes being left empty. I don’t want us to be the world’s capital for money laundering. I want to give first dibs to Londoners.”

In 2013, 15% of new homes in London were sold to foreign buyers, according to research by the British Property Federation and Molior, a consultancy.

City hall officials have calculated that last year the lowest number of affordable homes was built in London since records began in 1991 – just 4,880. Khan said he wanted to build 50,000 of all types of housing a year.

The plan to dramatically increase the amount of affordable housing could set the mayor on course for a high-stakes haggle with private housebuilders who, after eight years Ken Livingstone demanding up to 50% affordable in the developments, enjoyed a more lenient regime under Johnson.

Some large property developers are willing to build more affordable homes, but they have also told Khan’s team they want certainty about what they are being asked to deliver. Tony Pidgley, the chief executive of Berkeley Homes, said in March that if authorities demanded 30% affordable housing, developers would respond by lowering land values to accommodate the lower profitability. But 30% would still not be enough for Khan.

“There are some pieces of land where we want 50% affordable if not more,” he said. “Globally, I want half of all new homes to be genuinely affordable.”

But the industry body the Home Builders Federation (HBF) says greatly increasing the affordable housing target in London could backfire and end up reducing the overall supply of new homes, as developers decide to scrap plans or sit on sites until a new administration came in with more modest goals.

“Levels on each site have to be realistic,” said a spokesman. “If he sets the target too high he will potentially see a reduction in overall housing supply at a time when we are only building half of what we need already.”

The HBF said it was “trying to open negotiations with the mayor” and urged the mayor to focus on releasing underused public land.

Khan also plans to settle rows between councils and housebuilders about how much affordable housing could be built before a scheme becomes unprofitable. His team is working on creating a single system of viability assessments and to make the previously secret calculations public.

Viability assessments on the 681 home development of the former Royal Mail sorting office at Mount Pleasant in central London varied considerably. Royal Mail said only 24% of the homes on the planned redevelopment of the site could be affordable while Islington and neighbouring Camden council complained that 42% was possible and at a lower rent than proposed by the developer.

“I am keen to make viability assessments transparent,” said Khan. “Some developers hire consultants and run rings round local authorities and we want to stop that happening.”

He also confirmed he was planning to create a city hall unit to plan, finance and build new homes. He said Homes for Londoners would use vacant Transport for London sites as well as other publicly owned land. He added that major employers could be asked to provide finance and in exchange they would get new homes for their workers.

“I have met many chief executives and those in multi-nationals who say we have problems with recruitment because of the housing crisis in London,” Khan said. “My point to them is this: join Homes for Londoners, you can provide some of the finance and we can guarantee some of the homes we build will be for your staff.”