In general if the garage was just converted to a bonus room with no additional plumbing or electrical then you should be fine. Certainly, the appraiser would need to comment that the conversion was done in a workmanlike manner with no health or safety issues. My company works with many lenders, but one of the more strict lenders is B of A and they would require this to be appraised as a garage with cost to cure noted. But, if the addition has additional electrical or plumbing then permits are required. Other investors will not necessarily require permits if there is new electrical or plumbing but will mostly depend on the appraisal and appraiser comments in regards to this.
To summarize, if you think the home will appraise for what you are paying without the garage conversion, then that is the first hurdle. And as long as the conversion poses no health and safety issues, you have a good chance of this working out for you.