For agents
The whole sales comparison approach rests on one question: what else could this buyer have bought instead? Answer that well and the adjustments are small and the value is defensible. Answer it badly and no amount of arithmetic afterwards will save the number.
Most people reach for proximity first — find the nearest sales, then work with what turns up. That is backwards, and it is the single most common reason two people price the same house differently.
A comparable sale is useful in proportion to how completely it could have substituted for the subject property in a buyer’s mind. A house three streets away that is the same age, size, storey count and style is a better comparable than one next door that is half the size and forty years older. Distance is a proxy for similarity, and when you can observe similarity directly, you do not need the proxy.
Every adjustment you make is an admission that the comparable was not quite right. Fewer, smaller adjustments mean a better comp — and a value you can defend when somebody pushes back.
It is not a feeling. There are specific characteristics where a difference changes which buyer pool the property sits in — and those matter far more than the ones that merely change the price a little.
Living area, lot size, bedroom and bathroom count, garage spaces, pool, age and condition, view, and the date of sale. These are differences in degree rather than in kind — the same buyer would consider both properties, and the question is only what the difference is worth.
Notice that square footage sits in the second list, not the first. It is one characteristic among many, which is exactly why dividing by it and multiplying back does not work.
The honest answer is that it depends on how fast the market is moving, not on a fixed number of days. In a flat market a sale from nine months ago can be perfectly usable. In a market moving quickly, a sale from three months ago already needs a market-conditions adjustment, and one from last week is worth more than one from last spring.
The failure mode is using an old sale without acknowledging the date. The second failure mode is refusing to use an excellent older sale in favour of a poor recent one. A well-matched sale from six months ago with a date adjustment beats a badly matched sale from last month with four large adjustments.
None of these disqualify a sale automatically. They are reasons to weigh it less than a cleaner one — which is a different thing from throwing it out, and a much more honest way to work.
When a seller disputes a price, they are almost never disputing the arithmetic. They are disputing the comparables — “that one backs onto the road,” “that one hadn’t been touched since the nineties,” “mine has the bigger lot.”
Which means the conversation you need to be able to have is about which sales you used and why, not about the number at the bottom. A valuation that lists its comparables, scores how well each one matches, and shows what was added or subtracted for each difference gives you that conversation. A single number gives you an argument.