Is it permissible for an employee to dispose of leftover building materials after the completion of projects, given that their food and accommodation expenses were at their own personal cost, unlike the other employees?

If the company explicitly or implicitly permits you to benefit from the remaining construction materials, then you may sell or use them. Otherwise, it is not permissible, due to the Almighty's saying: "And do not consume your properties among yourselves through falsehood," and due to the Prophet's (peace be upon him) saying: "The wealth of a Muslim person is not lawful except with his willing consent." As for food expenses and housing rent, if the contract explicitly states that the company bears them, then this must be fulfilled. Otherwise, it is based on custom. If your right to this is established and the company does not fulfill it, then you may obtain it by selling these materials or by any other method through which you can secure your right without harming others. This is known as the issue of al-Zafar bi al-Haqq (seizing one's right).