Economy of Scale – When a contractor is building many new houses at once, materials can be purchased in large quantities for delivery as needed, driving prices down.
"Top-Down" vs. "Bottom-Up" – New construction begins at the foundation and builds upward. Repairing a house that isn't totally destroyed often means removing the roof and rebuilding from the top down, a far more labor-intensive procedure.
Use of Labor – When a new home builder has several houses under construction, work can often be scheduled for the most efficient use of carpenters, plumbers, and electricians. When only one home is being built, that kind of efficiency is rarely possible.
Access to the Worksite – With new home construction, there is usually no landscaping, allowing easier access to the building site. When a house is being rebuilt, there are often trees, shrubs, lawns, and other obstructions limiting access. Materials may have to be off-loaded and hand carried to the site, which increases labor costs. Condo/Co-ops also have their own access challenges, including parking issues, elevators, dumpsters and more.
Special Features & Unusual Materials – Older and extensively remodeled homes often include customized features and uncommon materials which can be expensive to duplicate. Examples include slate or tile roofs, lath and plaster walls, coved ceilings, custom ironwork, ornamented fireplaces, exposed beam ceilings, stained glass windows, curved staircases, and slate or tile roof.
Building Code Changes – Building codes have evolved over the years to meet higher safety regulations, adopt to new energy efficiency technologies, and respond to the environmental demands of extreme weather. If you are rebuilding or restoring your home, newer, more demanding codes may apply to your home, including undamaged parts of the structure. You may need to replace windows with safety glass or replace roofs with fire-retardant materials. These changes can add thousands of dollars to the cost of a home.
Construction Costs Rise After Natural Disasters – After a disaster such as a hurricane or fire affects a wide area, the costs of building materials and contractor fees often rise sharply due to the surge in demand. Local supplies are quickly exhausted, so materials have to be brought in on an emergency basis, often from mills or factories at great distance. Skilled labor is also in demand after an event impacting a neighborhood or region. This increases the overall coos the work as well as the timeline of the project.
Undamaged Parts of the Home Must Be Protected – Once the fire is out or the windstorm has abated, property that has not been destroyed must be protected from further damage or looting. This can involve covering a damaged roof. There’s an expense and labor cost related to quickly protecting  and securing the home and it’s contents from further damage.