An appraiser is going to compare other comps to determine the the value of a particular characteristic (called linear regression analysis).
In other words, find several recent 1-bath comps and several recent 2-bath comps, and compare them determine the actual market value of a second bathroom (or some other property characteristic) in your market, then adjust for that.
In other words, with everything else held constant, if 3-bedroom 2-bath homes sold for $15k more that 3-bedroom 1-bath homes, then the market value of that second bath in three bedroom homes is $15k.
They may also consider the cost approach.
If the average cost to add a second bath to a 3-bedroom home is $20k, they might assign a certain weight to each approach and value a second bath at $16 to $18k (somewhere between the market value and the cost), for example.
When you are learning to value and compare properties yourself, it can be very helpful to learn the basics of appraisal techniques, and get your hands on some actual real estate appraisals from your market to see how a professional appraiser actually makes these adjustments.