The Prime Minister branded sceptics of the mortgage guarantees - such as Liberal Democrat business secretary Vince Cable - "London-centric".
In many parts of the country prices were "barely moving at all", he insisted.
More than 6000 people have put offers in on homes and applied for mortgages using Help to Buy since it was launched about three months ago.
Nearly 750 homeowners have completed their purchases and hundreds were able to spend Christmas in their new homes, the Government said.
Mr Cameron said the initiative meant people without wealthy parents and a big deposit could "realise their dream" of owning a home, and rejected concerns that high demand was already overheating the market.
"Where we are today, house prices are still way below the peak they reached in 2007," he said. "Forecasters do not think they will get back to the level before the crash even in 2019. So there is no evidence of a problem.
"This is about helping people to achieve the security and stability they want of owning a flat or a home of their own. We are not helping people to buy flats or homes they cannot afford.
"We are helping people who do not have wealthy parents, who cannot get a big deposit together, and helping them to realise their dreams."