"After years of explosive growth in the Phoenix area, the apartment market has begun to cool," says David Wetta, the regional manager of Marcus & Millichap. "Although Phoenix may have experienced a crescendo in terms of vacancy and rental growth last year, strong market fundamentals will keep demand elevated."

One of the key indicators is that vacancy rates are inching upward all over the Valley. At the start of the year, the overall vacancy rate was 7.4% and has climbed nearly a half percentage point through the first half of the year.

Rent growth is stagnant in a sharp turn from prior years when it spiked as much as 5% to 8% in some submarkets. In the next 12 months, rent is forecast to rise and average of just 3%.