There are two parts here.

1) building more units/houses does increase capacity, of course. But unless capacity in a location exceeds the population of residents AND everyone who would want to move there (doesn’t happen in a desirable place), ‘empty’ units will be rare. That is assuming price based backpressure doesn’t exist of course.

It’s a bit like the freeway capacity fallacy. Adding more lanes to a freeway, until you exceed the capacity of anyone who would ever want or be able to use it, just makes traffic worse, as it becomes a more and more known artery and additional businesses/people start using it, which increases traffic.

NYC housing prices are still astronomical, for instance, despite them being on a housing building binge for at least a century.

2) since a ‘limited’ (non infinite supply, supply < available demand) housing location will generally have more than 1 person interested in every house for sale, and will always have a point where housing costs money, the gating factor for if someone wins a pricing competition for a house is their ability to pay more than everyone else.

That means more leverage allows people to push prices higher.

It isn’t just interest rates of course - underwriting standards play a part too. Someone who can’t document employment history can’t get a mortgage, for instance.

If you consider the ability to leverage a force multiplier - say 30x for someone who can get a 30 year loan, but It’s actually more, depending on downpayment requirements - then those able to use the leverage will outcompete those who can’t.

those who can’t meet underwriting have a leverage factor of 1x - they can only offer the cash they have on hand. No leverage.

If we wanted to make housing more affordable (but didn’t mind throwing the US and world economy into a black hole), we could for instance make it illegal to issue mortgages. Then as long as you could save money, even if you were a drug dealer with no documented income, you’d be one the same playing field as someone with the high tech salary. You might lose still of course.

When people had to pay cash for a house, the typical house was around 1-3x the typical personal yearly income at the time. The reality is that a ton of people still rented and were homeless though, but that is a different discussion.

As to why this means ‘cheap money’ vs ‘expensive money’ means there is a good/bad affordability impact, even when the funds rate should (in theory) just change some multipliers in a calculation, but not if someone can actually buy or not.

The part you’re missing is speculation, time, and cycles, which hasn’t come up yet.

When money is cheap, underwriting gets looser (but not loose! No one is writing a mortgage for someone homeless, even at the top of the boom without going to jail.). They do this because the loan originators need to compete for buyers.

When prices of an asset go up, and money is cheaper, there is an initial lag - people don’t think of the asset as a ‘sure thing’ because it hasn’t been growing year over year yet. They won’t lever as high. Underwriting standards are often still tight. They’ll be conservative.

This is when things tend to be more affordable/in reach.

If someone is stretching to pay at 15% interest, historically interest is lower than that too. So when rates drop, they can upgrade, and prices go up, so they get money off it too. They can get a bigger and better place. Or move somewhere nicer.

This starts raising prices, but it takes time.

As this starts happening, underwriting standards start loosening. After all, if prices have done nothing but go up the last 10 years, of course they’ll keep going up, right?

As prices show this upward trend, everyone starts speculating too. If money is still cheap and getting cheaper, they can use leverage effectively too. Why buy one house, when you can buy two after all? Especially when prices keep going up. You don’t want to miss out.

Some folks also start doing things like NOT selling that first starter home, and still buying the second home. And renting out the first one. They were able to refinance the first home at the lower rate after all, and are making more at work, so why not?

This decreases supply of for sale houses of course.

As things get hotter and hotter, the amount of leverage goes up. This prices out more and more people in the market, aka folks with less money and less ability to meet underwriting.

It doesn’t decrease demand though - there are more risk takers buying more, as they continue to escalate as they see their asset valuations ballon.

People start doing their forward projections using the last few years numbers, and holy moly. Let’s double down, we’ll get even richer!

Houses for sale get richer and richer offers, with fewer contingencies, all to beat the other guy and actually close the deal.

At some point though, something happens. It literally can’t go up forever, with ever increasing profits.

When that happens, folks start recalculating their projections using new numbers. Numbers that don’t show exponential forward growth.

And that often means all the deals that were happening only because people DID project that growth become untenable.

But sellers don’t give up that easily, and since they’re still cash flush and they’ll be rich if they can sell at the price they expect, they hold on as long as they can.

Which adds potentially years of ‘stuck’ prices. Depending on how wealthy the area is, it can be held up for 3-5 years, with zero volume closing, but lots of listings.

Poorer neighborhoods it’s usually much faster.

As things sell at less astronomical prices, that is a dip in the curve. If it’s short lived, it won’t change much.

But people after awhile start revising their projections. And a lot of deals stop making sense after that - for people and investors. When someone is going to stretch and be ‘house poor’ when prices are going up, they are very uninterested in doing that when prices are dropping. After all, they’re locked into that mortgage for 30 years, and can only refinance if interest rates are lower.

That pushes demand down a lot.

As mortgages written when money was cheap have issues (due to lax underwriting), and as dealflow shrinks due to overall shrinking number of actually closing deals, underwriting standards tend to tighten too, making it worse for new mortgages, and decreasing available leverage.

This takes years to play out however. ‘07-‘09 were strong contraction years, for instance.

Sellers start being interested in folks who aren’t levered so high (their deals will close). They might entertain contingencies, because they have to. They’ll have to drop the price, not sell, or be repossessed and have the bank take the loss.

All of these are great for sane people who can avoid being taken advantage of.

More supply from existing stock comes on the market, as it’s untenable to just keep it off hoping for gains. People who were previously making crazy living situations work stop doing so, moving people to less dense living arrangements.

Jobs start shifting around too, often to lower cost of living areas, as markets cool and it’s less worthwhile sticking it out in an overheated area.