Jane Martin said they must take account of all the dimensions of any proposed property to make sure it is not ‘materially larger’ than the one it might replace.
The warning follows the ombudsman’s ruling against St Helens Council for allowing a five bedroom house to replace a dormer bungalow in green belt.
The council was told to apologise to the two sets of neighbours who complained and pay each £250 to recognise the time they had spent on it, their trouble and frustration and loss of confidence in the authority.
Dr Martin said councils must apply their policies consistently and the fair treatment of all planning applicants was ‘fundamental to good administration’.