Is the apartment, which the questioner participated in building in his father's house, his property?

The apartment is not your property by virtue of your participation in its construction with your father. However, if you did so with the intention of claiming reimbursement from him, then you may claim what you spent on it, or if you participated with him in its construction on the understanding that it would be shared between you, then it is according to what you both agreed upon.

However, if your action was a voluntary contribution, then you are not entitled to anything, and you will be rewarded for it. It is important to note that your residence in the apartment does not make you its owner unless your father grants it to you. In that case, he would then be obligated to grant a similar gift to the rest of your siblings, because justice must be observed among children in giving gifts, and it is not permissible to favor one of them unless there is a legitimate reason; for the Prophet, peace and blessings be upon him, said: "I do not bear witness to injustice."