The M&S has this whacky section called "C Yard/Unit Costs". I like to measure the driveway, count the number of small bushes, medium bushes, large bushes, and then do the same for the trees, I'll also figure out grass cover to estimate the cost for sod (if that is typical) or seed. I'll do this for 10 houses a year and figure out what the price of the landscaping and trees are on average per square foot for a particular quality, and apply that.
I will include the cost of site improvements in the section as well, based on their contributory value in the market as found in the sales comparison approach. Fences have no value in just about every market I am in, so that is an easy skip 99% of the time; the pools are in there, wells, septics (which can be extracted through vacant land sales in my area with well and septic compared to those without them - a plethora of data in most places). I know, a lot of people think you can't extract from sales data for the COST approach, but the reality is, we aren't figuring out "Cost" in the sense that these are the raw costs to build a dwelling, we are determining "Cost" in the sense of "Price", what someone is willing to pay a builder for building this house on a square foot basis. To me, the cost approach is, therefore, just a different version of the Sales Approach.
When it is all said and done, I plug my numbers in. I don't show my work in my reports for where the site value came from. This is a summary appraisal for goodness sake. I am not going to include the last ten of these I did and show the data. My reports are long enough. Maybe I'll put "M&S Sec C plus pool, well & septic", on the site line, but that's it.
I assume my peers know this section well enough to determine if my numbers are reasonable.