Outlet pipe should always be slightly lower than the inlet pipe, sounds to me like your 'Builders' have scratched the surface, thrown a few lengths of pipe in and kicked the soil back over the top.
I would seriously recommend you get someone who knows what they're doing in to sort this out properly, before you find you've sewage backing up towards the house and are in an unholy mess, and are unable to even flush the WC.
I would seriously recommend you get someone who knows what they're doing in to sort this out properly, before you find you've sewage backing up towards the house and are in an unholy mess, and are unable to even flush the WC.