reading a lot
Most people read a lot off the zoning. Look up the district, do the math, and that's the unit count. That number is a ceiling. What the lot actually yields is decided by things the zoning map doesn't show, and most of them can be found from a desk in about ten minutes.
I read them in order- the cheapest question that can kill it goes first. If an early one fails, you don't need the rest.
1. is it a legal lot
Platted, or not. A tract that was never platted, or was split off without approval, has to be fixed before anything gets permitted, and that is time most pro formas never carry. You can't see it on a site visit. The plat is public. Look for it first.
2. what sits on top of the zoning
An overlay, a neighborhood plan, a deed restriction, an HOA. Any of them can rewrite the envelope, and then the base zoning is the wrong answer. Deed restrictions and HOA rules aren't on the city's map at all- you have to read the documents.
The city doesn't enforce deed restrictions. Austin approves plans that meet its code whether or not they meet a deed restriction- the people named in the restriction enforce it, and they can do that after the house is built. The city may not enforce them, but your lawyer neighbor, bound by the same restrictions, certainly will- and can sue you over a house that doesn't conform. Read them before you buy, not after.
Some deed restrictions stop the owner from building something in their own back yard. That's the wrong way around. Near the city center we want that second unit- it's how urban neighborhoods densify and how we slow sprawl. If you choose not to build one, that's fine. Being prevented from building one is the problem. And because the city doesn't enforce deed restrictions, you can get the permit, build it, and still be taken to court over it by whoever the restriction names.
3. water, both kinds
Is any of it in a floodplain? Check the city's layer, not just FEMA's- they aren't always the same line. Then which watershed it sits in, because outside the central urban watersheds that can cap impervious cover below the zoning, and impervious cover often sets yield before the zoning does.
I once looked at a floodplain lot in the city. The planner I talked to knew the address by heart- it had come across their desk several times, and so many people had looked at buying it that the low-hanging fruit was long gone. Proving the drainage wouldn't hurt the lots next door, designing in the floodway, and building on what was left was a steep hill: engineering and approvals well past an administrative sign-off- in Austin, a floodplain variance goes to City Council. I sketched it suspended above the floodway, with the parking right on the road, the highest part of the lot. Nobody had built it, and there was a reason.
4. can it be served
This one isn't on any map. Ask the utility about capacity before you have a unit mix, not after. One more bathroom can cross a fixture-count threshold and upsize the line all the way back to the street- more on that here.
And check who already holds the street. Utilities and internet providers can hold a work zone on a block for a long time. If one already has a permitted work zone on your block, the city treats your permit as a conflict, and you have to resolve it- by schedule, a shared traffic control plan, or their release. On the bigger streets the city wants that in hand before yours is issued.
5. what can't move
Heritage trees, a creek, an easement, a building old enough to trigger review if it comes down. Most of these can be designed around. None of them should be found after the survey is ordered.
a test fit isn't the read
A lot of developers do early test fits in-house now, fast, with AI tools. A test fit tells you what shape fits inside the setbacks. It doesn't tell you whether the lot is legal, whether the line to the street is big enough, or what the deed restrictions say. Those come from somebody looking at the lot and getting into the granular details.
the one rule
Find out everything you can without spending a dollar: the county records, the overlays (neighborhood and transit), new ordinances, state legislation, upcoming hearings on density. Then spend money, in order, before the big loan. I'm all for spending money early on a lot- once the free questions are used up. Your time is cheap here, and so is the risk.
the point
The zoning is the ceiling. The lot decides how close you get to it, and most of that can be known before anyone draws anything.
For the first part on an Austin lot, how many homes fit on this lot now reads the HOME rules for one lot.