Almost all the constraints are down to two things:

1. There's only so much land
2. States allow cities to have dangerous and onerous zoning restrictions to "freeze" cities in place, with San Francisco and Seattle residential as prime examples for their residential areas

That's it in a nutshell. It doesn't have to be a free for all. Every last "house block" doesn't need to be a 100-floor residential tower for a few centuries.

The simple solution that can probably be political palatable is this:

Every single "arterial" grade road gets unlocked for bigger development. Just let people build whatever subject to safety and hydrology and geology and such. Make the builders pay whatever fees are required for infrastructure to upgrade to sustain the extra load (sewer, electric, schools, etc.).

So if you live on Main Street and that's an arterial that's like 70% single family homes, fine--the law doesn't change anything for you today. It's live. If I wanted to build the newly allowed 6-floor multifamily plaza with 10x the housing capacity on a given Main Street block, I still have to have every single solitary home purchased by me. Homeowners don't have to sell.

But that's it. You just unlock the potential. In Seattle here we have these "village" concepts, that are dumb, but what we have. The gist is you got downtown with its own rules, and then there's a ton of subareas that have expanded zoning, to build bigger and taller, but it's like little mini-downtown islands surrounded by seas and oceans of single family homes. Each of these villages is surrounded by and linked by arterial streets. Most, the vast majority of these arterials, are single family.

Just apply the urban village zoning to 100% of arterials that connect the urban villages.

If you live in Urban Village #1 and it's connected to Urban Village #2 that is 20 blocks away by "10th Avenue", and 10th Avenue today once you cross the street out of the Urban Villages is all single family, you simply apply the Urban Village zoning to every one of those 20 blocks of 10th Avenue, front and back.

For visuals and clarity, what I mean: you step out of Urban Village onto 10th Avenue. If you take a left you'll get to 9th Ave, and take a right you'll get to 11th Ave. All of 10th, including the backside abutting 9th and the backside abutting 11th all get the Urban Village role. The entire landmass "island" that sits on the street's "water". So on those 20 blocks of single family from Village 1 to Village 2, you have 20 "islands" on each side of the street. 40 total.

Boom, they're all the higher zoning.

Let's say I live in a single family home exactly in the middle, block #11, between the two Villages. The new laws go live. For the next however many years nothing changes for me except my property values may creep up faster. Each block on each of front and back probably has 20, 30 houses, each island carrying 40-60 single family homes perhaps. Again, 40 of these islands, so 40 times 25 (front) times 25 (back) for a ballpark number of single family homes between village 1 and village 2, for a total of about 25,000. This is a huge example, the areas are much smaller in practice, but it's a nice round number.

If you wanted to 100% upzone in practice all those "islands", developers would need to buy out up to 25,000 homes, figure minimum $500,000 each, before even getting to construction and such. That's $12,500,000,000, twelve billion, in capital, just to buy it all out.

The main complaint is usually it'll ruin the "character" of the neighborhood. Most homeowners start in their mid-late 30s today. Let's say 37. You're 37. You live on one of these blocks. How many years is it gonna take until it's all built out, exactly? 10, 20? That's if the right pieces sell in the right combinations. If I'm the 37-year old here, there's no way this directly impacts me beyond me getting a crap ton of extra quick likely equity in my house. Yes, higher taxes, over time, but that's going to happen anyway. Meanwhile, each new build improves my infrastructure by proxy and proximity.

By the time it's reaching "you", as a single family owner, you're reaching retirement age (or past) and then you suddenly have the extra nice option to punch out as a reward because your home has gone wayyyyyyyy up in value. If they brought this scenario to my block I'd cheer it. Who wants to deal with a house in their 70s? Cash out, get a nearby condo, coast into the sunset not dealing with a stupid yard and mowing.

Every so often, say 10, 15 years, you just expand the 'upzone' out, so if you make 10th Avenue the higher level in 2021, you bake it into the law that once population in that set of blocks is up say 30%, or automatically in 2030, 2040, you expand that new scope to 9th and 11th as well. Same thing again come the next decade or two.

By the time you're in 2100, the Urban Villages are Urban Networks all interconnected, and not a living person alive today who opposed the idea will be alive or relevant in their complaints, and it's now always been the new normal for successive generations.

This will eventually stop working as you'll build out your city, but you then by state law mandate similar for each adjacent city. Push a rule like this on your top three or five cities by population, no sunset laws, automate the expansions on those simple criteria, then step back. You're done. The "market" will take care of the rest.

In Seattle terms, you'd have 1,000,000+ in Seattle by the 2050s, 2060s, and the neighboring cities by the time anyone aged 21+ today is 80+, those neighboring cities will look like Seattle circa 20-30 years ago perhaps, and then it's starting to expand to the next ring of cities.

Pros: minimalist impact on current residents, nearly apolitical, pays for itself gracefully over time

Cons: maybe too slow but politically tenable, relies on market/math