>And you can buy a decent TH for ~$600k (that's "affordable" for DC metro).
That's not affordable, not even for DC metro. Houses in Greenbelt (one of the nation's original walkable planned communities) can be had for less than $400k. $600k is "large colonial" money, or "walk to UMD's campus" money. Even those numbers are not necessarily comfortable swings for the median worker.
The median household income in Greenbelt is significantly less than Reston ($82k vs $135k).
The average home sale for all of DC Metro, for all home types, is right around $600k. And average new homes in Fairfax County is around $1 million.
That's why I had "affordable" in quotes - it's not cheap, no debate there, but it's reasonable for the area given the demographics (and you can spend less, but get a less desirable home design or location). Lots of teachers and SAHM/Fs in my neighborhood, which you're less likely to find in Ashburn or McLean.
AFAIK it's never been $600k. NOVA prices have always been inflated and overweight the average (for super-fun reasons I won't get into but that you can probably guess), not a norm that MD prices fail to reach. This mortgage calculator (https://themortgagereports.com/mortgage-calculator) pegs a $600k home at ~$3700/m, which is going to be about 45% of the post-tax income (assuming a take-home of about $100k from that $135k), or ~1/3 of pre-tax. Depending on who you ask, that's just on the edge of affordability; it leaves no wiggle room, particularly if you've seen or will see transportation or food or other costs rise (so it makes sense that the actual median is lower).
It's perhaps advantageous for some to project the idea that a $600k median is reasonable, because they derive certain privileges from both the rising figures and the delta with what people in the area can actually afford comfortably, but I guess my point is that it's not actually reasonable. It puts a strain on household budgets and redirects funds that should be going to things like infrastructure, disproportionately affecting the less well-off. (It's also a bit of a feedback loop, as higher housing values can be borrowed against, and the debt used to bid up the prices of other goods.)
That's not affordable, not even for DC metro. Houses in Greenbelt (one of the nation's original walkable planned communities) can be had for less than $400k. $600k is "large colonial" money, or "walk to UMD's campus" money. Even those numbers are not necessarily comfortable swings for the median worker.