Surface rain water under my back extesnion

I recently had a side and back extension built and decided to pave the back garden by 50%. We have quite a bit if clay in the ground and a soakaway was installed. We have rain water from a rear garage going straight into this soakaway and we have rain water from the pitched roof of the extension also going into the same soakaway crate. We have drainage channels going right across the back of the house for the paving surface water. What we are experiencing now is when it rains heavily there is water under the extension we built and since it has stopped raining heavily the water level under the house has reduced but still there. We never had this problem prior to having this extension built and we had all the surface water from the roof of the old conservatory where the extension is now going straight to a public sewer. So could our problem be caused by: 1. A lot of building material being dumped by the builders into the grounds of our garden causing blockage. 2. The clay ground we have in the back garden (although prior to the extension being built never had a problem, and also currently our next door neighbours who do not have an extension do not have the same problem as us). 3. Soakaway not installed correctly. 4. Soakaway not suitable for clay ground and will make matters worse. 5. Too much pressure put on the garden with all rain surface water going in it from all the rain and surface water. (Prior to the extension the rain water on our old conservatory was going straight into the public sewer. I could get a Civil Engineer to look at it but I think they are very expensive so thought I'd try the forum just in case someone has experienced something similar.