Summary
Bath and garage rates are small, they move with house size, and nobody tests them, so they end up absorbing error from the other lines. Derive them from your own sales, treat the half bath convention as a convention and not a finding, and check them after the GLA adjustment, not before.
Why the small lines deserve attention
Nobody challenges a $2,500 bath adjustment, which is why it rarely gets checked. These lines get carried over from the last report more than any others on the grid. The numbers are small enough to look harmless and familiar enough to feel settled.
They matter for two reasons. First, they add up. Three or four small unsupported lines across three comparables comes to real money. Second, they move with living area, so an unsupported bath rate doesn't stay in its own cell. It counts a difference you already captured somewhere else, and it does that on the comparables that differ most.
Deriving a bath rate
The method is standard. The hard part is the matching.
- Match on living area first. A comparable with an extra bath is usually bigger too. If the pair doesn't hold size close, what you measured is size. Call it a bath rate and you count it twice as soon as you also adjust for square footage.
- Group by bath count inside a size band. Sort your sales into narrow living area bands and compare medians across bath counts within each band. That controls for size without needing a perfect pair.
- Regression with both variables in. Where your set supports it, putting area and bath count in together estimates each with the other held still, which is what you need here.
Expect the value to taper off. The step from one bath to two is priced very differently from the step from three to four, and one rate applied across that whole span overstates the top end. If your data shows the taper, band the rate.
The half-bath convention
Half baths usually get adjusted at half of a full bath. That's a reasonable default, and it is a convention rather than something measured.
What buyers actually pay for a half bath depends on how many full baths the house already has. A powder room in a house with one full bath solves a real problem. The same half bath in a house with three full baths often solves nothing, and your pairs will show it contributing little. Where you have data to test it, test it and report what you found. Where you don't, the convention holds up as long as you say that is what it is.
The same goes for three-quarter baths. Boards record them inconsistently and they are often mis-entered in the MLS. Verify the count on the sales your rate depends on.
Deriving a garage rate
Garage varies more than bath, because what the market pays depends on what is normal for the segment.
- Price per space, not per square foot. Buyers count bays. Derive a rate per square foot of garage and you get a number that has nothing to do with how the market thinks.
- Attached, detached and carport are different features. They shouldn't share a rate. Fold them together and the result describes none of them.
- The local norm sets the shape. Where two-car garages are standard, going from none to one and from one to two both get paid for, and the third bay contributes far less. Where three is standard, that shifts. Measure it rather than assume it.
- Being below the norm hurts more than being above it helps. A house with no garage in a market that expects one usually shows a bigger discount than an extra bay shows a premium. One rate applied in both directions will be wrong in at least one of them.
Check these lines last
Order matters. Time-adjust your sales. Derive and apply the biggest physical adjustment, which is usually living area (see deriving a living area adjustment). Only then ask whether bath count and garage still show an effect of their own on what is left.
Often they show less than you expected, because the size adjustment already picked most of it up. That's the right answer, and the right response is a smaller rate or a documented zero, not the number off the last report. How to write that zero is covered in sometimes the right adjustment is no adjustment.
The test that catches the double count
Look at the range of your comparable indications with all the adjustments applied, then again with the bath and garage lines removed. If taking them out tightens the range, they aren't measuring anything of their own and they are working against your reconciliation.
It takes two minutes and goes straight at the most likely error in this part of the grid.
Common questions
Can I reuse a bath rate across assignments in the same market?
A rate you derived recently in the same segment is a reasonable starting point, and it should still be checked against the current set, since that data is already in front of you. What you can't defend is carrying a figure forward without knowing when or where it came from.
How should I treat a bathroom in the basement?
The same way you treat below-grade space generally, and separately from above-grade baths. Count it like a main-floor bath and you overstate it in most markets. Ignore it and you understate it.
What about a converted garage?
It moves two lines at once: garage count down, living area up. The market often prices the conversion below the sum of those two adjustments, especially where a garage is expected. That's worth a sentence rather than two mechanical adjustments.
