Do the expenses incurred by the son in building a residence for his deceased father count towards the legitimate inheritance?

If the dwelling belongs to your father, then it is an inheritance (tarkah) to be divided among the heirs according to Islamic law (shar'an). What you spent on the house, if it was a donation (tabarru') without the intention of seeking reimbursement, then you are entitled to nothing more than your legal share of the inheritance. However, if it was with the intention of seeking reimbursement, then it is a debt owed to you by the estate (tarkah), which you should receive before the division. This applies if the heirs confirm your claim; otherwise, you will be required to provide sharia-compliant evidence (bayyinah shar'iyyah). Disputes are settled in sharia courts due to their ability to hear from all parties, whereas a mufti only hears from one party.