The "ID Code", or Swedish personal number, belonging to the villain posing as an electrician is faulty. The combination of numbers is invalid, which can be easily checked using the last digit.
The design of the house the way it is ultimately refurbished would have required multiple planning permission processes. There is no way that that could have happened during the timeframe of the film. The added exterior design would have drawn the ire of other neighbours and most likely resulted in a charge for building without permission.