We have lived on our new home for almost 5 years now. We have never been able to get hot water from our front bathroom tub and shower, but the sink water definitely gets very hot. I was talking with a neighbor that had the same problem and they caught it before their 1 year was up to get the building contractor to fix. They tor into the wall twice, but could never figure out what was wrong. Why can't I get hot water? And is it going to cost a lot of $ to fix?

Well, you are going to have to tell us a lot more than that.

Start with what kind of tub and show valve do you have?
Single handle or two handle? Picture would be nice.

I am sure there is hot water pipe going to your tub/shower. However, today's faucets have anti-scald device that is built into the faucet. These anti-scald devices are factory adjusted to you only luke-warm water. You make the final adjustment yourself. If you tell us what kind / brand / type of faucet it is, we will tell you how to adjust that ring so you get hot water. Also, you can snap a pic of this faucet and post it for us to see. Back to you. Milo

It would also be a good idea to check for an isolation valve or stop valve on the faucet body that may have been left off or partially closed,( or a valve in the wall/ceiling that was covered. Let's hope not)
Until we know if you have a single handle, pressure balanced faucet it's worth a check.
Lesson to be learned for those buying new homes. Don't just live with something that is wrong when your house is completed. Some builders bank on cheap labor and materials to make it past the one-year-mark so they don't have to do warranty work.
My money would be with Milo. A scald guard that needs to be adjusted is usually, but not always, the culprit. Sometimes, in new construction, the valve body is soldered without removing the cartridge. If you have copper pipes this may also be the problem as damage can occur with too much heat.
Start as Milo stated... with a photo unless you know the make and model.

I attached a pic of the faucet. There is a screw that looks like you can adjust it with an allen wrench? Is that it?