You need professional help with this issue, its not an amateur project.
It may be a safety issue. Take care and have a structural engineer involvrd.
You need a foundation contractor to evaluate the problem and resolution.
This is not a diy project. Please call a professional for this.
Might i suggest you contact Local Authority about this one as I am in UK and don't know the answer for your area...............sorry!
Foundation problems happen here in Texas because of our clay soil and hot weather. This is definitely not a job for DIY, if your foundation is cracked. If you are just talking about a gap around the house, get soaker hoses and place approximately 6 inches from the house. Water once a week in the winter for about an hour. In the summer you need to water more often to keep the ground from shifting. By doing this you keep the soil damp so then it does not dry out and crack and move away from your house.
If you have foundation problems, doors stick, doors don't Open And Close Properly.
Gaps Around Window Frames Or Exterior Doors.
cracks in the corners of door frames. Sagging Or Uneven Floors. Then call a foundation company to check it out. Hopefully this is not the case and you caught it in time!
Foundation fixing cost approximately $15,000 on a 1800 foot square house.
My architecture firm’s TX Projects have a large amount of their Clients’ Construction Costs in properly mitigating Expansive Soils.
Since I have no On/Site data to advise you, I would remind you that you probably know your neighbors “irrigate” their foundations.
If your foundation is already broken and crushed, I advise you to:
1.) Trench Now, as it is wet weather season in Tx.
2.) get under your house and document the damage, as you will need it in communicating a Scope Of Work, in order to get Apples to Apples Bids.
With 30 years of experience in both Expansive & Liquefaction Soils Remediation, I have more intellectual property if you should want it.
This issue is very serious and you need to consult with a structural engineer