Delaware has no statewide residential building code and therefore publishes no statewide frost depth: the Delaware Code gives each of the three counties its own building code chapter, so the frost line a footing must reach is set by New Castle County, Kent County, Sussex County or the incorporated town that issues your permit.
Delaware leaves the frost line depth to the local building department, which is where the number for an address comes from.
The County Council, in order to provide for the health and safety of the citizens of New Castle County, shall promulgate rules and regulations, to be known as the New Castle County Building Code, controlling, regulating and supervising persons engaged in construction activities and providing for matters concerning, affecting or relating to home improvement contracts and to the construction, use or occupancy, location and maintenance of buildings and structures located in New Castle County9 Del. C. Chapter 25. Building Code, Delaware Code Online, read September 23, 2026. Read it at the source
That makes the building department issuing the permit the office with the number, and it is a short call: the counter gives the depth for the address, and the same call settles whether a deck footing or a slab edge needs a footing inspection before the concrete goes in. What Delaware does publish is below, and it is worth having in hand first.
A frost line depth turns into two separate requirements in most codes: how far a footing has to bear below grade, and how far a buried water service has to sit below the same line. Delaware writes one of them out.
Plumbing is the one construction code Delaware sets statewide, and the state adopts the 2021 International Plumbing Code; none of the state's amendments states a burial depth or a frost depth, so the depth a water service must reach comes from the model code text as applied by the code official issuing the permit.
The Delaware Division of Public Health shall hereby adopt the 2021 International Plumbing Code with following amendments. The code shall be known as the "2021 State of Delaware Plumbing Code". Every political subdivision within the State, including county, city, or municipal governments, shall enforce the International Plumbing Code (IPC) as adopted or modified by the Division of Public Health.4455 Delaware Regulations Governing a Detailed Plumbing Code, Delaware Administrative Code, read September 23, 2026. Read it at the source
The air freezing index is the quantity the frost provisions of the model codes are written on: add up how far each day’s mean temperature fell below freezing over a winter, in degree days, and take the deepest run the winter managed. It measures how much cold the ground had to absorb, which is why it is the honest way to set one place beside another. Wilmington is the coldest of the 2 cities on the record in Delaware.
159degree days
average freezing index a winter
Wilmington New Castle Co AP
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Wilmington | Delaware | 159 | 530 | 96 |
| Dover | Delaware | 93 | 299 | 84 |
Computed from NOAA National Centers for Environmental Information, Global Historical Climatology Network daily temperature records for each station, from 1951 onward. These are measurements of winter, not depths.
Only a handful of states answer with one number for the whole state, and they are the only codes that can be set beside each other directly. Ranked by the freezing index at their own reference cities, with Delaware marked at 126 degree days for comparison against what those codes settled on:
Left to right, the mildest winters to the hardest. The figure on the right is what that state wrote into its code.
The nearest by winter are North Carolina at 12 inches, Massachusetts at 4 feet and Connecticut at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Delaware: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Delaware code read September 23, 2026 from 9 Del. C. Chapter 25. Building Code, Delaware Code Online. Codes are amended and a local amendment can be deeper than the state figure, so the building department issuing the permit has the number that binds. Every state is on the frost line depth hub.