To answer the title's question, I wish I could say, “Just because.”
I know that is not going to convince anyone though, so I will give some examples of why EVERY home needs an inspection in the context of a real estate transaction--even new construction.
While I agree the number of serious issues likely to be found in the context of the inspection of a brand new home goes down, how many “little things” would it take to equal the cost of an inspection? Even the most expensive home inspector’s fee will likely be well below the cost of a few minor things and sometimes considerably less.
On a recent inspection, where the client opted to not have a purchase inspection done, all I can say is they were mighty glad they got the one year warranty inspection done. With the builder still in business, some of the issue will now hopefully still be able to get resolved.
There were a few things I would consider cosmetic but for this discussion I will only discuss the things that will clearly add up to considerably more than the cost of the inspection--if the builder was not in the picture.
The first thing discovered was that the vent cap for the high efficiency furnace was plumbed backwards. Instead of the exhaust blasting away from the house it was instead directed toward the siding, as the exhaust came out the area where the air intake was supposed to be.
I noticed a slight amount of rusting of the fasteners of the cap (circled in red) and I would not have anticipated that much rusting in eleven months---especially on the non-weather side of the home. Firing up the IR Camera, it became immediately clear the cap was plumbed backwards.
The white/yellow area around the cap is the exhaust gases venting agains the siding.
The next significant issue was the 4 inches of water in much of the crawl space. This might be quite an expensive fix depending on what is necessary---including possible installation of a sump pump system.
There was also no pan under the water heater, the roof metal drip edge was missing in one location and on top of the roof underlayment everywhere else, and the blower door test results were 25% higher than they should have been.
All of these things would likely have been discovered had an actual inspection been done, but what if the one year warranty inspection had not been done?
I am by no means the least expensive home inspector in my area, and I have never not easily found at least the cost of the inspection in a new construction home.
Getting a thorough home inspection done is part of due diligence.
It should not be taken lightly.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board