We want to make use of the underground part of the house which is lower than the house' septic tank. Is it safe to bulid a septic tank directly under the concrete floor of the house and how is it safelydone under this condition?thanks

No you can not build a septic system under a house but you can install an upflush toilet. You can build an ejector pit with a pump to move all sewage up and out. This is not a DIY task for most folks so I'd get 3 estimates and then decide if you can do it or if you can afford a pro.