The first thing you need to do is check maps. What you need to find is the earliest map that shows a house on the site, then you have some sort of indication when it was built.
However this is fraught with difficulties because small and isolated houses are not shown on most maps before the First Edition of the Six-inch-to-the-mile Ordnance Survey maps in the mid-19th century. See
http://maps.nls.uk/view/74425411 , which dates from 1867.
Also, just because a house is shown does not mean that it is the house that is still there. It is surprisingly common for houses to be demolished, demoted to outhouses, or abandoned in favour of a new building.
This
http://www.geograph.org.uk/photo/465641 shows Cot-town farm house as it is today. It looks fairly plain, but there are one or two bits of embellishment, so I would hazard a guess that it was built in the earlyish 19th century, or maybe perhaps a little earlier. But I could be well wrong!
The best way to find out about the date of building of a house is from estate records. The Registers of Sasines record
changes in the ownership of land and buildings, not changes like erection or demolition of buildings, and you already know which estate it was so the Sasines are unlikely to tell you anything significant.
The best source of the information you want, if they have survived, would be the records of the Forbes estate. To find out whether these have survived, and if so where they are, start with the Scottish Archive Network
http://www.scan.org.uk/Musing further, Cot-town as a farm name is interesting. There are masses of Cot-towns, Cottons, Cottertons, Cottertowns and so on in Scotland. All of them indicate that they were originally cotters' houses, often attached to a farm of another name and occupied by the married farm labourers with their families. So it would be reasonable to suppose that a farm named Cot-town was originally part of another farm, but leter separated off as a farm in its own right. I speculate that this particular Cot-town might have been part of Waterside at one time. Again, the best source of information about this would be estate records and plans, if they have survived.