The deadlines are absolute
Two clocks start the day your sale closes. You have 45 days to formally identify replacement property in writing, and 180 days from closing to complete the purchase. These run concurrently, not consecutively — the 180 days is not an additional window after the 45. Neither is extendable for ordinary business reasons, and missing either one makes the entire transaction taxable. This is why replacement property search should begin before your sale closes, not after.
What qualifies as like-kind
The term is broader than most owners assume. Any real property held for investment or productive use in a trade or business is generally like-kind to any other — a medical office building can be exchanged into retail, industrial, multifamily, or raw land. What does not qualify is your primary residence, property held primarily for resale, and, since 2018, personal property of any kind. You must also use a qualified intermediary: if the proceeds touch your hands between sale and purchase, the exchange fails.
Common ways exchanges fail
Three mistakes account for most of them. Taking constructive receipt of proceeds instead of routing through a qualified intermediary engaged before closing. Identifying replacement property too loosely or too late, and discovering on day 44 that the target is unavailable with no backup identified. And trading down — if the replacement property costs less than the sale price, or you take cash out, the difference is taxable boot. Identifying multiple backup properties within the rules is the single most effective protection.