My toilet is about 30mm lower than the soil pipe, will it cause a problem? or do I need to raise the toilet height?
Lower the soil pipe definitely
Due to the water flushing through it should expel the waste, problem is the lowest point will always have sewerage sitting inside so over time it can build up, cause blockages or worst case back flow into the toilet pan.
It is most definitely a problem, it's essentially a negative head height, so yes the loo needs raising