Here's something most people who live here don't even know. Louisville has the largest collection of Victorian mansions in the United States. Louisville also has more shotgun houses than any other city in America. Number one in both. The fanciest houses of that era and the plainest, in the same city, built in the same thirty-ish years, a few miles apart.

That's not a coincidence. Between 1870 and 1900 the city doubled, to about 200,000 people, mostly immigrants working in tanneries, packing houses, distilleries, foundries, rail yards and tobacco warehouses. Nobody had a car. You walked to work. So you needed a place close to the plant, cheap enough to afford on a factory wage. Hundreds of local builders answered that with the same house over and over: the shotgun. A lot of them were built to be rentals in the first place. They've been doing that job for over a century.

If you've never been inside one: one room wide, usually 12 to 15 feet across. Three or four rooms deep, in a straight line. No hallway. You walk from the front room through the bedroom to get to the kitchen.

Two variations worth knowing if you invest:

• Double-barrel: two shotguns sharing a middle wall, side by side, two front doors. That's a duplex, and a built-in house hack. Live on one side, rent the other, and let your tenant help pay your mortgage.

• Camelback: a shotgun with a second story on just the back half. Gets you an extra bedroom on the same skinny lot.

I pulled the MLS this week. There are 119 single-family homes for sale in Jefferson County where the listing agent tagged the style as "shotgun." The median asking price is about $139,000. Seventy-three of them, about three out of every five, are asking under $200,000. (Agents choose the style tag, so the real number is probably a little higher.)

There are 22 single-family homes for sale in the Old Louisville preservation district. Median asking price is $409,000. Same city, roughly three times the price.

What I tell people to look at before buying either one:

Shotguns. These are 100-year-old houses. Bathrooms were often added decades later, sometimes not well. Look hard at the foundation, the wiring, the roof and the drainage. Small lot, small yard, small utility bill. And under $200K is the slowest slice of our market right now (sales down 22% from last year), so that's where the tired sellers are. Ask the seller to help with closing costs, ask about a rate buydown, ask if their loan is assumable.

Big Victorians. A lot of them are already split into two to eight units. Know going in that Old Louisville is a local preservation district, so changes to the outside of the house go through the city's Landmarks review. Budget time for that. Old houses also mean slate roofs, built-in box gutters, plaster walls, and sometimes old knob-and-tube wiring. And if you're restoring a qualifying historic building, there are federal and Kentucky historic tax credits that can give back part of what you spend. Talk to your CPA before you count on them.

What I love about this housing stock is that the houses never stayed on one side of the market. The 1895 limestone castle a tanner built on St. James Court became a boarding house, then a home for elderly women for about forty years, then a museum. Workers' cottages that were falling apart are now some of the most charming small houses in the city. One on Goss Avenue serves breakfast and pizza. The houses didn't decide what they were worth. The city around them did, over and over again.

For anyone who owns century-old rentals: what's the one thing you check now that you wish you'd checked on your first one?