There are different types of inbreeding. Someone that really knows what they are doing can get by OK for several generations, sometimes indefinitely, by being selective in picking their breeders. For people with my skill level and conditions, it does not work that way. One big element is how many different chicks you hatch so you may have more to choose from and how many different roosters and hens you have providing chicks.
You never know what traits your chickens have when you start off. It is possible you could have some defective genes that will really mess it up. Possible, but not usual. Most of us can do fine breeding chickens without introducing outside genes for three, four, maybe five generations, even if we are not experts. You do need to be a little careful in selecting which chickens you keep for breeding purposes, hens as well as roosters. Obviously don't breed chickens with physical deformities, like cross beaked or deformed feet. But also look at other traits a bit. If you want good egg layers, try to hatch chicks from hens that lay well. If you want meat, you might select a rooster that is bigger rather than smaller. And watch for behavioral traits too. Aggressive hens and roosters tend to have aggressive offspring. I can't tell you everything to look for, but just select for your breeders chickens that you want the offspring to be like.
The problem for people like me is that our flocks can lose genetic diversity after a few generations, especially if we don't have several different hens and roosters to breed. So it is usually a good idea to bring in a new rooster every three, four, or five generations to get new genes in the gene pool, especially if you only keep one rooster with your flock.
You are dealing with living animals so no one can give you any guarantees, but many of us do OK inbreeding chickens for our flocks. Good luck!!!
