Where they will get water? How they will get food? How many roads and transportation and wasted energy will be required to deliver these bare necessities? Where they will dump their shit and garbage? I sure hope - not in the Ocean! My wild guess, dude, is that you will still need rest of USA to feed and support that concentration of population. Not to mention need for mental health support, because of encroaching of personal and communal space. People might be very social animals, but they are not sardines.
>>Stop eating meat
Consumption of meat - in particular, cooked meat, only in 3 million years made primates into humans. In that light, abandonment of meat also can lead to evolutionary changes and lack of protein, that meat is providing will affect brains.
Not raising cattle mostly shifts land usage to single culture crops, which are no better for environment and lead to impoverishment of arable lands. The main reason why animals are raised is economy - it takes less effort and energy to raise animals, compared to money you would earn from raising crops - to reach the same amount of money by raising crops, you need more arable lands and resources to maintain them - not to mentioning, that raising that amount of single crops will decrease their price value(because of open market) that will affect all crop growers and in the end they will be impoverished - not some dude in CA, who dos not have to tend crops.
>>>More UK specific
In UK many highrises from 1970s are razed to the ground and in their place are rebuilt family size houses, because highrises are extremely expensive to maintain, not to mention, that in crisis situations they are unmanageable - if there is a fire, then death is in 10s - even hundreds, if there is a single crime, then affected is not just one household and neighbouring families but the whole highrise.
UK is transforming some of the lands of defunct mines into forests, because it is dangerous to build house in place, that can suck it up with all the inhabitants. Wetlands are contantly exposed to tides from sea - also not viable place to build a house. Well, you can build house anywhere - in space, on the botom of the ocean, even on the active volcano, but it all comes of unsustainable costs for most of us out there. No one is holding you back to do what you preach - on your own expense.
"Housing crisis" in UK is not about lack of houses in a market, but lack of social welfare houses, that can be offered to people, who are living on welfare - and the main crisis here is that such people share in UK has reached significant share among population.