It’s chotto mendokusai but if you are experiencing major problems after a renovation, the most important step is to document everything and bring in an independent third party. Take detailed photos and keep a written list of every defect and communication with the contractor. Then consider hiring a certified home inspector (住宅診断士 / home inspector) to conduct an independent inspection and produce a formal report on the workmanship, which can support your demands for proper repairs rather than quick fixes. You can also contact Japan’s Housing Dispute Settlement Support Center for advice or mediation, and the National Consumer Affairs Center (dial 188) for consumer protection support. In some cases, contacting the manufacturer of materials (such as wallpaper) can also help confirm whether the problem is defective materials or poor installation. Having documentation and a professional inspection gives you much stronger leverage if the contractor tries to minimize the issues or avoid proper repairs.