What do raises have to do with a flat tax? If everyone was taxed at say 10%, 10% of their raise no matter how much it was would go to tax. Raises shoudl be an incentive or rewerd, while taxes are an obligation, two completely separate things.
A flat property tax would simply tax everyone the same % as the property is worth. Someone owning a million dollar mansion would pay 10 times the amount someone owning a 100 grand house. Not exactly rocket science and not that far from how it works now.