The average home owner underestimates their monthly running costs by around £700, while the typical renter underestimates outgoings by nearly £600, research has found.
Renters put their monthly household outgoings at £1,227, which is £592 less than the national average of £1,819, according to Zoopla's calculations.
Zoopla took mortgage and rent costs into account to make the calculations as well as water, energy and council tax bills, using figures from the Council of Mortgage Lenders (CML), uSwitch and Government data.
The survey of more than 2,700 people also found that more than one in three (37%) bill-paying Britons admit they have no idea of the monthly cost of running a home.
Older people and younger people were particularly likely to say they do not know their monthly running costs, with 43% of over-70s and 42% of people in their 20s saying they were unaware.
One in nine (11%) people said they do not pay attention to monthly direct debits, and a fifth (21%) said fluctuating bills add to their confusion over monthly running costs.
The findings were released as Zoopla launched a new running costs tool on its website to give people an estimate of how much it could cost them to run a property each month.
Lawrence Hall, a spokesman for Zoopla, said: "House-hunters could risk a nasty surprise by not factoring ongoing expenses like energy and water bills into their budgeting."