travel expenditure- employment income

there is an employer who feels that his building needs restoration and some repairs, which would get done within 24 months, so he decides to move his office temporarily to neighbouring building. Now all his employees get mileage allowance as long as they come from their homes and drop by their temporary office. But my question is what if after 18months the employer realises that it is going to take 12 more months(30months in total) before the old building is completely prepared to be reoccupied, so the employee will get mileage allowance up to 6 remaining months, or stops receiving any car mileage allowance due to fact that the temporary workspace no longer qualifies as temporary.