how to gague how much a house is worth after rehabing

[Samuel Chua]

Good evening, I am a new investor who was looking through some houses to practise my cash flow calculations. However, it soon dawned upon me that I did not know how much a house would appreciate after rehab and how I can gauge the rent/price after rehab. Do I have to hire a 3rd party or do I use the trail and error method? Advice would be appreciated. Thanks!