What is the correct word to refer to a company's home area?
Hometown itself works for the purpose. Examples:
"This month, managers and executives from U.S. Security Associates (USA) are making plans to gather in the security company’s hometown of Atlanta, Ga. for the ASIS International 60th Annual Seminar and Exhibits." -- Security Today
"The catalogue is distributed both in stores and by mail, with most of it being produced by IKEA Communications AB in IKEA's hometown of Älmhult, Sweden where IKEA operates the largest photo studio in northern Europe at 8,000 square metres (86,000 sq ft)." -- Wikipedia
"We're proud of our Georgia heritage, and more specifically that we call Atlanta our hometown. Our headquarters, known within the company as the Support Center for restaurant Operators, is located just outside of downtown." -- Chic-fil-a
Home base -- M-W
(noun) 1. the place in which someone or something lives or operates.
"The company's home base is in New York."
I would choose locality as it is not as specific in meaning (definition) as other suggestions.
Good alternative is municipality as the link you provided has this:
Kielitoimiston sanakirjan mukaan käsite "paikkakunta" viittaa varsinkin kuntaan.
Translation:
According to Language office's dictionary, concept "paikkakunta" refers especially to municipality.
Base of operations. Merriam-Webster:
main offices, headquarters:
The company's base of operations is (in) London.
In your sentence:
Is your company's base of operations still Helsinki?
Note that "official" from your original sentence is redundant, as base of operations implies official recognition.
domicile
Definition of domicile from the English Oxford Dictionary:
The place at which a company or other body is registered, especially for tax purposes.