This data was collected from the US Census, which conducts a monthly building permit survey for every US metro. 

There are a ton of data centers in loudoun county and prince William.

It seems doubtful that there would be a sustained downward trend in values, simply because for Crestline and Running Springs especially you have people who live up there and commute down into the valley but if fire strikes the area it could be pretty bad short term.

@Jacques Edouard Dessejour LapierreThe appraiser is going to have access to MLS sales and other data.  

We then bought another property a single family in a better neighborhood but declining not growing in value. 

It's cool to see in such a booming city that no doubt has a lot of competition you guys are finding ways to invest and make money.It has its ups and downs, you can’t just go and buy a $30k duplex anymore, but on the flip side our properties just keep going up in value at an insane rate. 

Some data supports that my parent's generation (the baby boomers) are STAYING PUT as that generation is approaching retirement age.

I'm leaning more towards Northern VA due to the relatively friendlier landlord laws vs. those in DC, the job growth due to data centers, Amazon, and warehousing in that area, and the relative affordability. 

It sounds like you learned some invaluable lessons on this investment that probably saved/will save you from these kinds of situations on future investments! 

For one property I would check competition and adjust manually to have the absolute best pricing data.