Does pre-approval affect your credit score?

No, because the credit check conducted during pre-approval is a soft enquiry, and soft enquiries do not affect your credit score.

Hard enquiries may affect your credit score if you lodge a lot of them. Hard enquiries include applications for a loan, such as a vehicle loan or credit card loan. Too many such enquiries gives the bank the impression that you are taking on a lot of debt.

But as mentioned, pre-approval does not include a hard enquiry, and will provide you with your credit score so you can ascertain your chances of home loan approval. 

Pre-approval also provides you witn an idea of what you can afford, which in turn improves your chances of home loan approval as your prequalification certificate will show that you can afford the home purchase.