What caused the 1989 Toronto housing bubble burst?

Ah yes!  I remember it well.
 
In the late 1980s, Toronto was recovering from the recession of the early 1980s.  I myself had my first decent job and many of my friends of the same age (late 20s) were doing well enough that they could consider entering the real estate market.
 
However, this was also the time that the GTA as a whole was growing, with people coming from all over the country and the world to live in Toronto.  Due to complex reasons, the rental market in Toronto was saturated and little new rental housing was being built.  There was rent control, which meant finding an apartment was a real hassle.  Similarly, due to really archaic zoning laws and the maturity of the city, Toronto had pretty much been "built out" - there was very little space available for new housing. 
 
The only new housing that was being built was condos.  Developers preferred condos to apartments because they allowed a quick return on investment (get in-get out) and they could be built in the same zoning areas that apartments used to be built in.
 
Due to the demand, real prices for real estate started to rise.  This led to the two natural consequences of human behavior - greed and fear:
Greed - buy real estate and sell it later for a big profit
Fear - get into the real estate market before it becomes unaffordable
 
Most of my friends who "got in" at this point paid a very high price to do so.  They did so for two reasons - they figured that if they waited they couldn't afford it, and they figured that they couldn't lose money.  One of the reasons this was because that's how it worked for their parents.  Their parents struggled to pay $12,500 for a house in Metro and now those same houses were worth $200,000.  This was the obvious time to spend $200,000 for the same type of property.
 
However, something was rotten in the state of Denmark.  Although there were plenty of people around, most of the condos were being bought by people who were going to "flip" them once the condo was finished.  Many people had made money this way - they had committed to pay $70,000 or thereabouts for a condo and found it was worth $100,000 when they actually took delivery. 
 
The nice (and unfortunate) thing was that you could reserve a condo for next to nothing - a $2,000 deposit.  You didn't worry about the down payment requirement because you never planned to take possession. 
 
Then, of course, the inevitable happened. 
 
Despite the growth in population and the need for housing, there were actually too many condos being built.  Condos are great for some people, but if, for example, you have a family of four, they won't suit you.  One of the reasons big houses sold so well is that a lot of immigrant families bought a five bedroom and housed the father, mother, two grandparents and five kids there. 
As a result, when the condos were finished, the number of avaiable buyers was lower than expected.  This meant people who put down a deposit were stuck with taking delivery.  However, some of these people had three or four units lined up and couldn't close.  They tried to walk away and found the downside - if their $70,000 unit had to be dumped for $50,000, they were on the hook for the $20,000 deficiency.
 
In reality, a lot of developers found themselves holding the bag.  About half the new condo units were empty.  Developers rented them out, but they were still on the hook for condo fees to the new condominium corporation, and didn't have enough cash coming in to pay off the debts they incurred in development.  Developers went bankrupt.  Then condo corporations went bankrupt.  Banks and receivers took possession of hundreds of empty condo units and it was bargain basement time. 
 
The glut in the condo market drove down prices in the GTA real estate market.  House prices came back to earth - not cheap, but low enough that many mortgages were "under water".  Interest rates ticked upwards just as new home buyers had to renew.  The banks were friendly and covered the difference in most cases, but what this meant was that most people were trapped in their homes making higher payments with no guarantee that their house would ever sell for what they paid for them.
 
One of my friends actually went through this and when they had to move for work, they had to pay to get out of the situation.  They have done well since, but it was an expensive lesson at the time.
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