When planning excavation in New York City, project teams usually focus first on the buildings immediately adjoining the site.
But when a historic structure is involved, Chapter 33 requires the review to extend farther.
Under NYC Building Code §3309.4.4(2), historic structures that are contiguous to or within a lateral distance of 90 feet from the edge of the lot where excavation is occurring must be monitored during excavation, subject to the exception contained in the section.
That means a historic structure does not necessarily have to share a property line with the project to be relevant.
The Code Says “Lateral Distance”
The wording matters.
BC §3309.4.4 distinguishes between ordinary buildings and historic structures:
- For other buildings, the code looks at the distance from the edge of the excavation in relation to the maximum excavation depth.
- For historic structures, the code uses a 90-foot lateral distance from the edge of the lot where excavation is occurring.
So the historic-structure provision is not measured from the excavation cut itself.
Where Does TPPN 10/88 Come In?
This requirement also has roots in an older DOB policy that is still referenced today.
Technical Policy and Procedure Notice 10/88, issued by DOB on June 6, 1988, is titled:
“Procedures for the Avoidance of Damage to Historic Structures Resulting from Adjacent Construction.”
TPPN 10/88 established procedures intended to reduce the likelihood of construction damage to nearby historic structures and to detect movement or damage early enough for construction procedures to be adjusted.
The original TPPN also defined an adjacent historic structure using a 90-foot lateral-distance concept.
DOB continues to list TPPN 10/88 in its official policy library and references it in current guidance for shoring, underpinning, and protection of adjacent historic structures.
What Counts as a Historic Structure?
Historic status is not always obvious from looking at a building.
DOB's current guidance identifies historic structures to include, among other categories:
- New York City landmarks or interior landmarks
- Buildings within NYC historic districts
- Properties listed or certified as eligible for listing on the National Register of Historic Places
- Contributing resources within designated historic districts
- Structures considered historic under applicable state or local law
DOB also specifically notes that projects within 90 feet of an adjacent historic structure are subject to BC §3309.4.4.
That is why checking historic status should be part of early excavation planning.
What Does the Monitoring Involve?
The current Building Code controls how required monitoring is handled.
Under BC §3309.16, where monitoring is required by Section 3309, it must follow a monitoring plan developed by a registered design professional and acceptable to DOB.
The plan must be specific to the structures and operations involved and establish:
- Scope of monitoring
- Frequency of monitoring
- Acceptable tolerances
- Reporting criteria when tolerances are exceeded
TPPN 10/88 provides additional historical guidance on issues such as building movement, settlement, cracking, vibration, groundwater, temporary retaining systems, and photographic documentation.
The TPPN should therefore be read together with the current Chapter 33 requirements, rather than treated as a standalone modern monitoring plan.
Does the 90-Foot Rule Apply to Every Excavation?
Not automatically.
BC §3309.4.4 includes an exception for excavation 5 feet deep or less where either:
- the excavation occurs more than 5 feet from all footings and foundations; or
- where it occurs within 5 feet of a footing or foundation, the excavation does not extend below the level of that footing or foundation.
That is why historic status and distance are only part of the review.
The team should also understand the excavation depth, nearby foundation conditions, and whether the exception applies.
Why This Matters During Preconstruction
This requirement can be easy to miss because the affected structure may not appear directly connected to the project.
A qualifying historic structure could potentially be:
- One or more lots away
- Near a corner of the project lot
- Part of a nearby historic district
But in every case, the actual 90-foot lateral distance must still be evaluated.
If the condition is identified late, the project may need additional monitoring design, surveys, access coordination, or other protective measures before excavation can proceed.
That makes historic-resource screening a preconstruction issue, not something to investigate after the excavation contractor is ready to mobilize.
The Takeaway
TPPN 10/88 may date back to 1988, but the 90-foot historic-structure concept remains directly reflected in today's NYC Building Code.
For excavation projects, remember:
BC §3309.4.4(2) looks for historic structures that are contiguous to or within a 90-foot lateral distance from the edge of the lot where excavation is occurring.
Not from the center of the site.
Not necessarily from the excavation edge.
And the code calls it a lateral distance.
So before excavation begins, don't stop the surrounding-property review with the buildings that share your lot line.
Check what is actually within the applicable 90-foot area, and confirm whether the monitoring provisions and exceptions apply.
Code & DOB References
NYC Building Code §3309.4.4 — Monitoring During Excavation
NYC Building Code §3309.16 — Monitoring Plan
NYC DOB TPPN 10/88 — Procedures for the Avoidance of Damage to Historic Structures Resulting from Adjacent Construction
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