In the dictionary both have the same meaning of "no access" or "no entrance". Are they interchangeable or is there any subtle difference between the two.
- kein Zugang – no access
- kein Zutritt – no entry
Kein Zugang means you can not access or reach a certain target this way – but it does not imply that it is forbidden for you to go there.
Kein Zutritt means it is forbidden.
Kein Zugang zum Messegelände
means: you cannot reach the Messegelände on this way.
Kein Zutritt zum Messegelände
means: you could reach the Messegelände on this way, but you are not allowed to.
Derives from treten in the sense of stepping. It implies physical access of a whole person. That is, the person as a whole reaches the place, steps on to it, into it, enters it or does something similar. For example, although I cannot step, when sitting in a wheelchair, I can have Zutritt to a facility.
Nur Militärpersonal hat Zutritt zu dieser Einrichtung. / Only military personnel may enter this facility.
As rightly remarked in a comment, this generalisation (to wheelchairs) should not be carried too far. A boat does not have Zutritt. We may use Zugang.
Die Englische Flotte machte der Türkischen den Zugang zum Hafen unmöglich. / The English Fleet prevented the Turkish one from accessing the harbour.
The reason I stress access as a whole is that there are verbs such as Zugriff. That describes physical or abstract access, but without these restrictions. It relates to greifen, grab in English. I can have physical Zugriff to some weapons without moving more than my hand!
Er hat keinen Zutritt zum Waffenlager, aber Zugriff auf alle Pistolen / He has no clearance for the armoury, but access to all pistols.
Indicates much broader access or reach. It may concern immaterial access:
Nur Militärpersonal hat Zugang zu diesen Daten. / Only military personnel has access to these data.
Zugang derives from the verb "gehen". This verb can mean walk, but can have a broader meaning, like the English go can. The spectrum of meanings for Zugang is indeed broad. In a business or sports team a Neuzugang is someone new to the team. He has recently reached the team.
On their own, I'd consider them interchangeable. Theoretically, you could argue that "Zugang" ("access") has a somewhat broader meaning than "Zutritt" ("entrance"). "Zutritt" refers to bodily enter somewhere, where "Zugang" can more generally mean "access". So you could say something like
Kein Zugang zum internen Firmen-Netzwerk von diesem Terminal.
which would translate to
No access to the internal company network from this terminal.
or, in better English,
The company's internal network can't be accessed from this terminal.
But I've never come across that broader meaning in just "Kein Zugang", only in longer expressions or full sentences.
As pointed out, both are used more or less synonymously, so never trust that the user meant the same thing you do.
Additional to the "you physically can´t / you aren´t allowed to" meaning, there are some additional hints:
"Kein Zutritt" is mostly used for closed spaces (i.e. buildings) while "Kein Zugang" could also describe an open place, e.g. a garden or park.
While "Kein Zutritt" always means "You are not allowed to enter." and describes the object you might wish to enter, "kein Zugang" could describe the access path or way ahead of you. It could mean "This way will not lead to the building, please use the next one further left.".
"Kein Zugang" and "Kein Zutritt" can both mean that you are forbidden to enter. See examples from the real world of signs:
As the Duden explains, both words have the meaning of entering (Betreten): http://www.duden.de/rechtschreibung/Zugang and http://www.duden.de/rechtschreibung/Zutritt. "Zugang" is even listed as part of the explanation of this meaning of "Zutritt":
1. das Hin[ein]gehen, Eintreten, Betreten; Zugang
So, with regard to this meaning, both words are synonymous. All claims to the contrary are simply false.
Our Server room has two lists 'Zutritt' for the admins that are allowed to enter the room to work and a 'Zugang' list of other people allowed accompanied access. The boss is allowed in but the zugang is restricted to 'in the accompany of someone on the Zutritt list'. Zutrittliste= Those permitted unrestricted access Zugangliste= Those allowed entry providing a condition has been met.