When a "building condition survey" is performed at the time of a real estate transaction of an existing house, does the seller pay the survey cost, or does the buyer pay it?

Either the seller or the buyer can pay the cost. In other words, which party pays the cost is decided through consultation.

However, if the buyer conducts and pays for the survey, the surveyor needs to enter the seller's premises or the interior of the building, so the seller's permission for the survey is required in advance. Please note that if you conduct an investigation without the seller's permission, the seller may notify the police of your illegal act.