I follow what many Appraisers look for:
4 Sold/closed escrow and 2 Pending comps within Â½ mile of the subject property and within 3 mos. (within + or - 15% SqFt.) I also make adjustments for pools, view, or any other special characteristics.
The next distance would be Â½ to 1mi. After 1 mile the Appraiser must explain complications of finding relevant comps, which I do not do, but as Heather comments, some "Artistic License" does come into play with highly unique properties.
As far as quality of data you do need to be working with a RealtorÂ®, as Anna suggests. Here's some background information on why:
"MLS Data Accuracy â€“ Where to search if youâ€™re â€œwithout RealtorÂ®â€
It depends on the property truthfully. If I am looking at a unique property then my distance may change to incorporate more comps. However, most of the time I will only look maybe at a mile radius around the property. But in addition to distance I look at properties that are similar in sqft, bedrooms, lot ize, year built, etc. I would recommend that if your looking at comps to purchase to ask your agent to run comps before making an offer. I wish you the best of luck!