Wood damaging insects such as termites, carpenter ants and wood bees allowed to continue their activities over a long period of time could do tremendous damage. Careful inspection for this type of damage is a must. Rodent, pigeon droppings and other animal offal must be considered very hazardous material and handled with care. The viruses and other agents associated with rodent droppings and urine are deadly. So use all precautions when dealing with an area suspected of containing this material.
Mold and moisture intrusion. Black mold and other fungus are byproducts of moisture. If an area has had high moisture intrusion over a period of time, expect mold and fungus organisms to be present. Careful inspection of moisture prone areas such as basements, bathrooms and kitchens must be a high priority. Many insurance companies will not insure if mold is present. Mold will not survive in areas that have humidity below twenty percent, but problem areas such as an old basement may have a difficult solution.
Urea-formaldehyde foam insulation (UFFI) installed in the wall cavities of older homes as an energy conservation measure. Many older homes may have had UFFI insulation installed as a cost effective way of insulating these older homes. The foam easily pumped into un-insulated cavities where it reacts chemically and expands to fill the cavity. Urea-formaldehyde is a known carcinogen.
Radon Gas occurs naturally and may be present in basements. Special sealing and venting measures may be necessary to bring excessive levels within government safety specifications. Radon is undetectable withoout special test procedures
Structural concerns: The structure of a building is essentially the skeleton and foundation that keeps a building standing up and solid. If the structure fails, then the building falls down. Older homes were built different than the homes of today. Many of the building system used in those days may be unfamiliar to many general contractors. But, engineering is engineering, so, a structural engineer should be able to spot any problems with a home's structure.
Balloon and timber framing, which was commonly used in the first half of the 1900s, may need considerable rework to meet existing building codes and fire codes.
Lumber used in early homes was typically rough-cut true dimensional lumber, meaning, a 2x4 really measures 2 x 4 inches and framing was not as uniform as today's framing. So, considerable rework of interior ceilings and walls may necessary for new drywall to lie properly.
Many older high-end homes have tile or slate roofing. While the lifetime of the tile or slate materials itself is essentially over a hundred years are more, the nails that attach the material to the roof structure may fail and the roofing material may become loose and fall. Removing and re-attaching the tiles or slate can cost many thousands of dollars.
Footings and foundations in older homes may be built from brick or stone instead of concrete. I have seen some foundations built from very large stone slabs. If the foundation and footings has lasted a long time without shifting it is probably pretty solid and not going anywhere, but in any case have it thoroughly inspected.
Electrical systems will almost certainly need revamping. Turn of the century knob and tube wiring will not be up to modern electrical code and safety standards. Exposed knob and tube wiring may be damged by rodents or other gnawing critters. Since, designers and architects may not notice these things during the design process, you need to be aware that these often overlooked systems may open up a big can of worms that might sidetrack a project later on.
Ancient outdated heating systems are for the most part unreliable/unsafe and should be replaced. If the house has an air conditioning system it will usually be a retrofit and may not be well installed. Count on re-vamping these systems.
Old plumbing systems can be a misery all it's own. Old pipes corrode and close up with minerals cutting off water and sewage flow. Count on many repairs. Lead piping was fairly common before 1950 for supply lines from the meter to the house and some is still in use today. Water setting in a lead pipe will become contaminated with lead over several hours. If you suspect that you have lead water supply pipes, you should let the water run several minutes before using it for drinking or cooking. Letting the water run through the pipes temporarily flushes the pipe with clean water. Also, consider using a very good filter system that will remove lead and other contaminates. Refrigerators that automatically supply ice and water should be on this filter system.
Including every aspect of old house remodeling is not humanly possible. Just remember to keep an open mind, communicate with your contractors on a regular basis and you will survive to live in a beautiful, likely one of a kind, early day mansion.