Not really - in London (and the rest of the UK) over the last few years large corporate landlords have been building huge developments of thousands of apartments, called the "build to rent" sector. It's unlikely all of these developments would have went ahead without them.

Even in the 'normal' development sector, landlords buy a significant proportion of the apartments available from new build developments. I would estimate that 30-40% of the apartments in my building were bought by landlords ahead of the development starting. This adds liquidity to the market and allows developers to build knowing they are taking less risk on selling them when the building is (more) complete.

I own a condo in Boston. Someone new comes to Boston and sees there are 100 rental units on the market. I decide to move and become a landlord. Someone new comes to Boston and now sees 101 rental units on the market. By moving away and becoming a landlord I've put another rental into the supply side.

> By moving away and becoming a landlord I've put another rental into the supply side.

The supply didn't increase. Demand reduced, because you moved away.

By your logic, if I stop eating hamburgers I've increased the supply of hamburgers.

That's a pretty good example of near negligible impact.