I going through the same process more or less. You should definitely contact them as soon as possible to get the ball rolling. You will need to have your degree and transcripts apostilized, then translated by UBA. You have to schedule this, which is why it is very important to start early.
Also, you will most likely have to deal with the CBC:
http://www.cbc.uba.ar/index.html
It looks like you would be in the Facultad de Arquitectura, Deseño, and Ubanismo, whose requirements are:
http://www.cbc.uba.ar/dat/cbc/materias.html#ARQU
You may be able to skip courses like "Introducción al Pensamiento Científico" etc. since you already have a degree, especially if your degree is in something similar. But as far as I know they are generally pretty strict about all UBA students completing the CBC. And, you can't take any course within the "carrera" until you complete the CBC.
But, as OutLaw said, only UBA can tell you the official answer.
If in the end, you find it isn't worth it, you might try enrolling in a MA program if they have it. They seem to be much less strict and will allow you to take courses in the "carrera" if you don't already have the background. At least that's how it *seems* to be in Filosofia y Letras.
Good luck and let us know what you find out!