The gas distribution company, İgdaş, whose head office/head quarters are at:
Eyüp Merkez Mah. Silahtar Ağa Cad. No:83 34050 Eyüp
lists one "general directorate service building" (not at the above address) and 17 "service buildings" on its website:
http://www.igdas.com.tr/Content_View.aspx?MI=6&CMI=516&MCI=4...It is a statutory requirement for a company to have a single head office/headquarters, so "head office/headquarters" will not fit the bill. It seems to be mainly public and municipal entities that have "service buildings". I would tend to go with Raffi here; apart from the literal translation, I do not see any other option.