Seriously, you don't. Construction really isn't my strength.
But let's say, for a moment, that I actually was a decent handyman - as a hobby. I coach, teach and train 5-6 days a week, but on my off times and in the evenings I enjoy building things. Back in university, maybe I worked a summer job helping framing, and nowadays get out to the garage to build birdhouse, a fence along the back... something along those lines. Now, you decide you want to build a house from scratch - you need to level the ground, put in the foundation, make sure it's structurally sound and will last for a century. Would you want to put all of this in my hands, or, although it's more expensive, would you hire a professional who makes their living at it, constantly educating and practicing what they do to elevate their craft?
If you'd rather hire me in this case to save money, well... what can I say. You'd get what you pay for. But if the thought of this is ridiculous, then let me ask: why would you hire a trainer who works full-time at another career, and trains people in their spare time?