I've built a small extension to my house with a concrete slab foundation, and now I'm noticing uneven settling and cracks in the floor after a few months. What steps should I take to address this issue?

Uneven settling and cracks in a concrete slab foundation often indicate issues with soil preparation, drainage, or material quality. Here's a comprehensive guide to diagnose and resolve the problem:

1. Assess the Damage: Inspect the cracks for width, depth, and pattern. Hairline cracks might be cosmetic, but wider cracks (more than 1/4 inch) could signal serious settling.
2. Check Soil Conditions: Ensure the subgrade was properly compacted and leveled. Poor compaction or organic soil can lead to settling over time. Consider a soil test to check for stability.
3. Improve Drainage: Water accumulation around the foundation is a common cause. Install or clean gutters, extend downspouts away from the slab, and regrade the soil to slope away from the building.
4. Repair Cracks: For minor cracks, use epoxy injection or concrete crack sealant. For larger issues, you might need to underpin the slab with mudjacking or polyurethane foam injection to lift and stabilize it.
5. Prevent Future Issues: Add a vapor barrier under the slab if not already present, and consider using control joints in future pours to manage cracking. Monitor humidity and water levels regularly.

If the problem persists, consult a structural engineer for a professional assessment to avoid safety risks.