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When thinking about a addition to a home one needs to be sure it's something for 100% they are going to do. I have had customers say we will be adding on to our home but don't realize the price if doing so. I can say honestly people that plan to add on out of all them. Only 15-20 percent to so.
So the question your asking is should I have my hvac contractor size the system bigger for possiable future addition? I would say this for what's it worth. I would size the home as it sets now and if you so add on to the home worry about it at that time. Why? Because you will have a oversized system to you do so that can lead to more problems till you add on.
With a addition only 300 square foot the installing of a mini split system would be perfect for that small of a area and if you don't add on you have not oversized your system.
As for equipment sizing you are refuring to add on of 300 square foot might be able to be cooled with your current system size if its real insulated and sealed with low glass area and or quality windows.
Load cal is the only way to no for sure. Contractor can run the load on the home as it sits and then preform another with the possiable future addition to the home. This will show him or you what the new add will require and what size system to install.
Again a word of caution... Watch sizing home for possiable additions that may or may not happen. As you be playing with fire that might not end up well if the addition is not added. Hints the mini split option I listed above. Perfect solution in this case because you plan to add on in 2-3 years if the system is sized for that then you will be oversized for the next 2-3 years.
