The decision means that the Building Society Ombudsman will be able to rule on complaints over property valuations, for example.
But if completion does not happen, the position will remain as it is now, because a legal transaction between the customer and the building society will not have taken place.
This will still stop frustrated would-be borrowers from making complaints against building societies in cases in which their valuers' reports have led to a mortgage appplication being turned down.
Michael Coogan, head of legal services at the Building Societies' Association, said he expected the change to affect relatively few complaints.
Until now, borrowers were unable to have their case considered if it related to actions before a transaction had taken place.
Yet people who already had a mortgage with that particular society were able to get their case looked into by the Ombudsman because they were already customers with a legal relationship with that society.
Mr Coogan said: 'There has been concern for some time about the gap in the service for customers who have a contractual relationship with a building society. This change will enhance the redress mechanism that already exists.
'If you have a mortgage and you find out your property is falling into a big hole, and think the valuer should have spotted it, you can complain to the Ombudsman even if the valuation clearly took place before a contract was signed.'
Mr Coogan stressed that someone with a complaint against a building society valuer because he had not obtained a mortgage, would not be able to complain.
Although the cases most likely to be considered were mortgage-related, he said that complaints over investments could be considered in the same way.
'It might cover a case where an investor called into a building society and was told about a particular rate of interest payable on a savings account,' Mr Coogan said.
'If he went into the society's office the next day and invested the money without being told that the rates had changed in the interim, then he might be able to complain.'Reuse content