Apple is once again rumored to be creating a standalone app to control HomeKit devices, possibly this fall with iOS 10. According to MacRumors, an Amazon reviewer who is supposedly a member of Apple's marketing team let plans for such an app slip earlier this week.
Some advice, there are many third party applications, most free, that offer more control and customization(s) with many Smart Home devices. "Yonomi" is a free app that I often use, "Home" is another which cost $14.99. Both offer support for many devices with more added daily (including Amazon "Echo"). The next version of iOS due this fall will have a standalone "HomeKit" app as well.
While it would be nice for Apple to release its own HomeKit app, the company has been rumored before to be releasing its own home-controlling application for iPhone and iPad, and nothing to that effect has materialized so far.
We might get a hint of Apple's possible plans for a HomeKit app in the company's announcement of iOS 10, which is expected to take place next month during WWDC 2016.