hi there marcus, if this can helps u would be gr8.
As a spanish native speaker, i would say that this sentence, "voy a la oficina" it's in simple present, indicative mode, and would be as the english sentence, i go to the office, lets see how it works in motion.
pedro, ¿dónde vas? me: voy a la oficina.
notice that even when we are not using a continuous form, a present one, it does have this semantic, as it is something you are doing at the moment of speaking, nevertheless, regarding grammar, the present simple is somewhat like english one, so mostly, we use present simple to talk about routines, habits, time tables, for instance:
Por la mañana voy a la oficina. probably talking about routines.
el lunes voy a la oficina. could be time table.
What you have to keep in mind marcus is that unlike english, in spanish we also use the present simple whenever we are talking in a continuous form, something u r currently doing. As an example:
¿qué haces santiago? Nada, hago mi tarea.
so it's sth u r doing at the moment of speaking.
Estoy yendo a la oficina is the continuous form, sth u r doing right now, would be "im going to the office".
Voy a ir a la oficina its like a decision u take, could be also an intention, for instance:
¿Ya has decidido donde vas a ir? Si, voy a ir a la oficina.
mmmm, creo que voy a ir a la oficina a chequear unas cosas.
But, the important thing here is that in spanish sometimes we do use present simple when we are talking about sth we are doing right now, like present continuous in english.
Hope it has helped u out marcus,