Assuming the policy to reduce non-primary home ownership is tax based, carve out tax exceptions for home builders. Personally I would carve out exceptions for home flippers too, but could see that being more contentious. Either way though, this part of the problem would be, IMHO, trivial to solve.

> If the average carry time goes up even a little bit (and it will because investors close faster) that can make whole developments untenable. Others build because they have large investors to sell to if necessary.

I think that's fair. But conversely carry time is also high because prices are high, and investors drive up prices; builders also do this to an extent, by e.g. buying down points to avoid lowering prices, to keep the perceived price elevated. I have no illusions this is a simple problem to solve. The right question here is probably figuring out whether the overall supply going up by X increased rate because of investors will be worth the cost of the total homeownership rate being controlled by a shrinking proportion of the population (I would guess no).

My personal proposed solution here would be to kick out investors (tax policy), and also directly incentivize home builders selling to first time owners (via tax credits, comparable to e.g. EV tax credits). The latter would be quite expensive, but if the overall homeownership rate increases and home prices drop, it would likely be popular.