I think you'll find that zoning/planning permission is the real bad guy here. That and a failure to understand Adam Smith and implement the ideas of Henry George.
Then you're a landlord who has purchased more of a scarce resource than they require (a house larger than you need) who has then turned around and rented access to the extra you have to people who can't afford a home of their own, and in so doing have driven the cost of homeowner-ship just slightly higher, which was the reason you couldn't afford it in the first place. Repeat that a few thousand times and that's a huge contributing factor to why housing is in such a dire state here.
You haven't solved anything. You just went from being an exploited person to being an exploiter instead, taking advantage of people you should have solidarity with and inflicting the harm the system was inflicting on you, onto them instead. The system will continue feasting on people who can't manage the same as you did, and you now posses wealth you did not earn.
Even if that wasn't the case, I don't see a problem with buying slightly larger than is necessary, because (for example) perhaps they're planning to have a couple kids in the next few years, but will rent out the extra space until then. Moving is transactionally expensive, and expecting someone to move every few years as their space needs change is unreasonable.
Regardless, you seem a bit overly judgmental about this entire situation, and about someone you don't know at all.
Landowners that have made improvements to the land and seek financial compensation for those improvements, in this case in the form of providing a service, is NOT rent seeking behavior.
That is NOT exploitation.
This is even basic economics from an extremely leftist POV, where those that have added labor value, that is improvements to the land, in this case providing a service and perhaps building a unit, managing them, etc. should be compensated for their labor.
Like this is extremely basic stuff.
No, I don't. Rent-seeking is derided behavior by basically everyone who isn't rent-seeking.
If you buy property, improve it, and sell it, there's your profit for providing that service. No ethical lapse whatsoever, unless you used that godawful gray laminate that every flipper uses. Then I'm mad at you still but that's a different reason.
If you own a thing that people need, and you gate access to it behind a paywall while maintaining ownership, and extract value from those people so they may use it but retain full ownership and control of it, that's rent-seeking and it sucks. You're the economic version of wind drag.
Yes, that includes the 98 year old lady who rents out a room to fill the gaps left by social security to the nice young man who's going to college in the area. Still value extraction. That young man is losing the value of his labor because he has to live somewhere and she has space he can live in. That's unethical.
If we, as a society, took this into account we would make it easier to build more housing.
The fact that we don't is the moral, ethical, and economic tragedy of our age.
Unless you were the very first person in the entire area to think to do so, then the existence of that very market for you to rent spare rooms on is actually driving up the prices of properties so you have to do so.
It's also driving up the prices of long term rental, because landlords make more money in the short-term rental market. The prices of long-term rental also affect the floor price of permanent ownership.
Consequently it is always likely that one might want at least one more room than is needed by the family. It additionally provides buffer overflow should the pitter-patter of tiny drains on one's resources appear.
In such circumstances renting out a spare room on a longish licence should necessarily reduce rents since now there is an option which is likely to be cheaper. Renting out a spare room for other people visiting ones city should similarly have had a depressive impact on the price of hotels or an improvement in their offering.
Given the inconvenience and reduced amenity one gains from living in a house under multiple occupation there is further no specific reason why it should be the case that this does not more than offset the cash remuneration meaning there is no in principle reason why the capital cost of such a property should increase.
The vast majority of AirBNBs are the entire house or some separate guest cottage. Many are owned and managed by larger companies. Or by people who own multiple houses.
The individual home owner renting out a room so they can cover their mortgage has become more of an outlier.
