The three tiers are what the phrase actually hides, and they are not interchangeable. Plain replacement cost pays the undepreciated cost of repair, but never more than the limit written on the declarations, so a policy insuring a house for $400,000 stops at $400,000 however much rebuilding turns out to cost. Extended replacement cost adds a stated cushion above that limit, commonly expressed as a percentage. Guaranteed replacement cost, where an insurer offers it, undertakes to rebuild the home regardless of the limit. The reason the distinction matters is that the limit was set from an estimate made before the loss, and rebuilding costs move, sometimes sharply and locally after a widespread disaster when everyone in a region is competing for the same builders. Whether the last two tiers are available at all depends on the insurer, the state and the property, and the terms of an extended provision are set by the policy rather than by any common standard.
The most important condition is that most policies do not pay replacement cost until you replace. The Standard Flood Insurance Policy states it plainly at Article VII.R.2.c: when the full cost of repair or replacement is more than $1,000, or more than 5 percent of the whole amount of insurance that applies to the dwelling, the insurer "will not be liable for any loss" on a replacement cost basis "unless and until actual repair or replacement is completed." Private homeowners policies commonly work the same way. The practical consequence is that a policyholder does not receive a check for the new roof and then decide whether to fit one. The first payment is made on a depreciated basis and the balance follows the work, a mechanism covered on the homeowners insurance page.
There is an escape route from that condition, and it has a deadline. The same federal policy provides at Article VII.R.2.d that a policyholder "may disregard the replacement cost conditions above and make claim under this policy for loss to dwellings on an actual cash value basis", and may then claim the additional replacement-cost amount later, "provided you notify us of your intent to do so within 180 days after the date of loss." That is a genuine option for someone who is not sure whether they will rebuild, and it is time-limited in a way that is easy to miss during the months after a flood. Private policies impose their own deadlines for claiming the withheld depreciation, and those are contract terms rather than a shared standard, so the number on any particular policy has to be read on that policy.
Replacement cost settlement is often conditional on how much of the property you insured. The federal flood policy allows it only for a single-family dwelling that is the policyholder's principal residence, and only where the amount of insurance is at least 80 percent of the full replacement cost immediately before the loss, or the maximum available under the program. Private homeowners policies use a comparable requirement, generally called a coinsurance clause, which reduces the payment on a partial loss where the property was insured below the stated share. That clause has its own page, and the arithmetic of it belongs there rather than here. What belongs here is the consequence: buying replacement cost coverage does not by itself guarantee a replacement cost settlement, because the amount of insurance carried is a separate condition.
It does not apply to everything on the policy, and assuming it does is the common error. Under the federal flood policy, actual cash value settlement applies to personal property, appliances, carpets and carpet pads, detached garages, two-, three- and four-family dwellings, and a single-family dwelling that is not the policyholder's principal residence. So a household with replacement cost on the structure can still be paid a depreciated amount for its furniture. Private policies differ, and contents replacement cost is often a separate election with its own price.
What it costs and what it buys. Replacement cost settlement raises the premium, because the insurer's expected payment is larger. The value of paying that premium scales with the age of what is insured: on a nearly new house with a nearly new roof the two settlement bases produce similar numbers, and on a house with twenty-year-old systems the gap can be most of the claim. That is the honest way to size the decision, and it is why the same choice is worth different amounts to different households.