I am allergic to pet dander and have asthma. i am moving to a flat where a dog lived, i only found out now. when cleaning there, my asthma flared up. there are no carpets no sofas nothing. should i look for another flat or will cleaning be enough?

Would not take the risk as dander and particles may remain in the AC vent ducts for extended periods of time, not to mention other possible particles. Specifically if your asthma already flared up, not worth taking the risk in my opinion.

Cleaning should use multiple methods, and I agree air filter HEPA units are helpful. Consider immunotherapy and please consult with an Allergist. Clean the apartment well, including air ducts and use a HEPA filter. Simple cleaning may not be sufficient to remove all the allergen present. Watch for signs of hives, sneezing, itching and wheezing. Keep an antihistamine and nasal saline available.