I have been digging my foundations for a week and keep having to stop and start due to building control issues. The site is next to woodland so we have had to go 2.2m down and have hit a coal seam. We've been told it needs to be dug out. Piling is out of the question as it's too costly and I was told it won't work in the area we live for various reasons. I've been in touch with a couple of structural engineers but keep getting conflicting information. Most recently I was told the coal isn't an issue and was advised to just back fill with type one and put a structural slab on the area for the ground floor extension to sit on and take the weight of the building. Is this the best solution?