This is The Housing Crunch, a five-part series on New York City’s affordable housing crisis.

When a lot of people compete for a small number of apartments, landlords often raise the rent — someone is almost always willing to pay more. This is the law of supply and demand in action.

That is one reason rents are so high in New York City, where just 1.4 percent of apartments were available last year. But there is another reason for the high prices: It has become quite difficult, time-consuming and expensive to build and maintain apartment buildings here. The costs are passed on to renters.

To help explain why, let’s put you in the shoes of a real estate firm. Imagine you want to build a large apartment building that has both affordable and market-rate units.

The Location

First, you need land.

You may not have as many options as you would think. You probably want your apartment building to be close to a subway station, and it would be great if it was in or near Manhattan, where most jobs are. Prospective renters will also want to be close to parks, restaurants, grocery stores and schools.