What knob-and-tube wiring is, and why it costs more
Knob-and-tube was the standard house wiring method from the 1880s through the 1940s. Hot and neutral conductors run separately, held off the framing on ceramic knobs and threaded through joists in ceramic tubes. There is no ground wire, no cable sheath, and the insulation is cloth and rubber that is now 80 to 140 years old.
Replacing it costs more than a standard rewire for three reasons. The old wiring has to be removed or safely abandoned, which is slow, dusty work in cramped attics and crawl spaces. New wire has to be fished through plaster-and-lath walls that resist modern fishing techniques. And K&T homes almost always need a panel upgrade at the same time, because the original 60-amp fuse service cannot stay.
Worked example: an 1,800 square foot K&T house
Take a 1920s colonial, 1,800 square feet, with live knob-and-tube on both floors and a 60-amp fuse box. At $10 per square foot, the low end is $18,000. At $20 per square foot, the high end is $36,000. That per-square-foot figure already folds in the removal labor, the new wiring, new outlets and switches, the panel upgrade, permits, and wall repair.
The low end assumes good attic and basement access, so the electricians can fish most runs without opening walls. The high end assumes plaster walls everywhere, knob-and-tube buried under attic insulation (which must be pulled back before the new work), and a full service entrance rebuild. Where your house lands between those numbers is mostly a question of access.
The insurance problem
Cost aside, knob-and-tube creates an insurance problem that forces the timeline. Many major insurers will not write a new policy on a home with active knob-and-tube wiring. Others will write it only with a surcharge or a written commitment to replace the wiring within 6 to 12 months. When you buy a K&T house, the rewire is not a someday project; it is a condition of keeping the house insured.
One nuance worth knowing: disconnected and fully abandoned knob-and-tube is usually acceptable to insurers. If an electrician can verify that every K&T circuit is dead and the new wiring has fully replaced it, you can often keep your policy without removing every last ceramic knob from the attic, which saves on the removal labor.
Can you live with knob-and-tube in the meantime?
Electricians are blunt about this: knob-and-tube in good condition, untouched, with no insulation packed over it, is less dangerous than folklore suggests. The real-world dangers are specific: insulation blown over K&T traps heat and is a genuine fire risk; DIY taps into K&T circuits overload conductors sized for 1920s loads; and the absence of a ground wire means no protection for three-prong appliances.
If you are living in the house while you budget, get an electrician inspection now, not later. The inspection identifies which circuits are live, whether insulation is contacting the wiring, and whether anyone has tapped into it. That inspection, usually a few hundred dollars, is the cheapest money in the whole project, because it tells you whether you have a year to plan or a problem to fix this month.
Getting quotes for a K&T rewire
Get three written quotes from electricians who do old-house work, and make sure each quote answers the same questions: is old-wiring removal included or is abandonment the plan, does the price include the panel upgrade, who handles the permits, and what happens with wall repair and insulation. K&T quotes vary more than standard rewire quotes because the access question is so hard to answer from the outside.
Ask each electrician how they handle the attic insulation question, because it is the single biggest source of surprise cost. If the attic has blown insulation sitting on knob-and-tube, it has to come out before the new wiring goes in, and that is a separate line item some quotes bury and others omit entirely.