The most expensive home improvement projects typically involve major structural changes or extensive renovations. Among these, a full kitchen remodel or a bathroom renovation can be costly due to the high-end materials, professional labor, and the complexity of plumbing and electrical work. Another significant expense is a home the design addition, such as adding extra rooms or levels, which requires architectural planning, permits, and specialized construction teams. Additionally, replacing a roof or installing new windows and siding can also be major expenses, especially if you opt for premium materials.