crime scene/place seized by police. when the person can claim its access?

As per the situation laid out by you, I wish to tell you that in case of crime scene investigation related to any crime, particularly murder, there are a lot of procedural formalities which need to be taken care of. One of them is to ensure that the scene of crime, that is to say where the crime has taken place, remains secure from any contamination so that necessary forensic evidences can be taken from the scene, which can help in identifying the cause of death, murder weapon, identification of the murderer etc. To that end, it becomes necessary to preserve the crime scene and keep it in that situation only in which the murder took place. In order to do so, the police seal the area and do not allow any unauthorized person to enter in the area. So, according to your question, the owner of the flat where the crime has taken place cannot claim possession of the property during the investigation.

However, once the investigation is done and there is nothing left to investigate in the crime scene, the police unseal the area. So, the owner of the flat can claim possession only after the investigation is done. To the best of my knowledge, no particular formalities are involved.