The National Audit Office raised concerns about the calculations behind the flagship housing policy and said the previous government had “applied a wide interpretation” of the plots that could be included in the tally.
It also criticised the target the government had set after finding no economic evidence to support it.
The report said the coalition had sold off enough land to allow 109,590 new homes to be built by the end of 2014-15 but warned that it had “concerns around the quality of the data” kept across Whitehall about the scheme.
It also pointed out the figures measure a “notional” number of expected homes rather than how many properties are actually being built and some developments could take up to two decades to complete.
By March this year the coalition government had sold 942 sites ranging from a plot with a single home to land expected to take 7,600 new properties.
The NAO’s investigation found that public sector land for 15,740 homes was sold before the 2011 launch of the programme yet was counted towards the 100,000 target.
Surplus land owned by Royal Mail was categorised as sold when the organisation was privatised along with sites owned by British Waterways when it became a charitable trust, even if the plots were not developed.
The watchdog also found there was “no consistent approach” in how deciding how many homes could potentially be built on a site.
Whitehall did not routinely track progress after deals were done and no record was kept of development activity, housing starts or completions, the NAO said.
The money raised from the sell-offs was not monitored centrally and it is not clear if departments have secured market value for the sites.