There are several ways in which you might translate this, but I suggest Vero pars aedificii interior in modum contra exteriorem indecorum pulcre ornata est. This translates the sentiment of the whole sentence, but in a way more natural to Latin in various ways, including the use of word order to imply the intended effect — for which it's often unnecessary to translate every word literally : other replies may be quite different, but equally valid.
Intus is an adverb, 'on the inside', etc. To use it isn't exactly wrong, but the inside of anything is generally pars interior, the 'inner part', which in this case I've used for contrast with the outside as (pars) exterior (in the acc. to follow contra). Vero is placed first in the sentence to emphasise the difference.