I am planning to build a garage on my property which would be completely below ground. the roof of the garage would be under the lawn of my garden. I need to know the best design or construction to ensure the build will be both watertight and structurally sound (the roof to bear the weight of the ground above and retaining walls) the approximate size will be (D)5m x (W)6m x 2.5m(H) with access to one side only the rest will be underground. I already have contacts for excavation and removal of earth but need advice on design and build techniques so completed project will be structurally sound.