What?
Premises and office costs include items such as renting and maintaining your building(s), utilities, computers, desks, furniture, etc.
In small organisations, staff are usually in one building, sometimes even in just one room. This means that the premises and office costs are shared among everyone, regardless of who works on projects and who works in other administrative parts of the organisation.
Premises and office costs are often an overhead cost for third sector organisations. However, sometimes some premises costs should be included as direct costs for some services. For example, suppose an organisation providing community programmes for the elderly holds educational classes in rooms in another building down the street. The premises cost of using the rooms (probably rent, and utilities if they are not included in the rent) is a direct cost of the programme since the rooms are part of delivering the programme. However, the organisation would still have premises cost for the building and rooms where all their staff work, which would be an overhead cost for the organisation.