The county was built outward from Washington, and then it stopped
Every parcel has a distance from the Zero Milestone in Washington. Take the median of that distance for each decade's new buildings and you get the position of the building line, decade by decade — a front advancing into farmland.
Where the decade's new buildings went up
Median and 90th-percentile straight-line distance from the Zero Milestone, Washington DC, for buildings completed in each decade.
How to read it: each dot is one decade. Higher means that decade's construction happened further out from the city.
How much went up
Buildings completed per decade. The two curves are the same county: one is where it built, the other is how much.
The 2020s bar is a partial decade — the file was assembled in 2025.
In 1980 the county drew a line around a third of itself
The Agricultural Reserve — about 93,000 acres downzoned in 1980, now the AR zone — was an explicit decision to stop building in the upper county. The tax roll records what happened next.
New buildings inside the Reserve, by decade
Counted against the AR zone as mapped today. The 1980s were the county's biggest building decade anywhere else.
What the numbers say
Caveat worth keeping: the boundary used here is the AR zone as mapped today, applied backwards to buildings that went up before it existed. That is the honest way to ask "what is inside the line now", and the wrong way to ask "what was legal then" — the 1980 RDT boundary was redrawn more than once on its way to today's AR.
The detached house stopped being the default
The assessor records a structure type for each building. Grouped into forms, they show a housing stock changing shape underneath a county that kept calling itself suburban.
Share of each decade's new buildings, by form
Detached houses, townhouses, condominium units, apartment buildings and everything else, as a share of what was completed that decade.
One caution the data forces: a unit is counted as a condominium if that is how it is owned today, not how it was built. Garden apartments converted in the 1970s still carry their 1960s construction year, which is most of the 1960s condominium block.
The median new detached house, in square feet
Recorded structure area of detached houses completed each decade.
The dip is real and it is the postwar starter house. The rise after it is not a correction — it keeps going.
Kensington, in one decade and change
The town was chartered in 1894 as a stop on the Baltimore & Ohio — a railroad suburb before there were cars to make one. Then the 1950s happened to it.
Buildings standing in the Kensington postal area, by decade completed
Every parcel the assessor files under a Kensington address. This is the postal area, which is wider than the incorporated town.
The oldest buildings still on the tax roll
Assessor's year built, oldest first, countywide. Each links to the state's record for that property.
| Year | Address | Place | In the historic inventory as |
|---|
Round years — 1700, 1750, 1800 — are assessor estimates, not documented completion dates. Where the state has actually surveyed the building, the last column links to the scanned survey form.
2,829 of these buildings have been written about
The Maryland Historical Trust keeps an inventory of surveyed historic properties. Each entry carries a scanned survey form — typed, often decades ago, by someone who stood in front of the building. Where one lands on a parcel in the tax roll, this page joins them.
Joined by position: an inventory record is attached to the nearest tax parcel within 60 m, one parcel each. The rest are districts, parks, canal segments and archaeological sites that do not sit on a single lot — they are on the map as inventory markers, but they have no parcel to join to.