Structural and Other Considerations for Vertical Expansion

04/08/2019 - White Papers


What does it take to add more floors to an existing building? 

How do I know if I can add more floors before buying an investment property?

Today we tackle these common questions in detail, so you’re better prepared before your next property acquisition deal.

The world of property acquisition moves fast. As the structural engineering lead in due diligence consulting for property acquisition, I’m in the trenches every day with real estate investors and their attorneys. As you might imagine, from this vantage point, I have a clear picture of the most common concerns surrounding real estate acquisition deals.

Today I want to delve into this common scenario:

In two weeks, my client is putting down a non-refundable deposit on a building.

How do we know now if we can add more floors to the building?

This is a loaded question.

Technically, the structural condition of the building can be observed in the field, and we can assess if the building is a candidate for vertical expansion. In other words, within 14 days, we can confirm if the structure is sound, needing minimal remedial work.

But what we can’t do in 14 days is confirm, with assurance, whether floors can be added.

Why is that?

While adding a floor or two doesn’t amount to Vertical Phasing (addition of 5 or more stories to an existing building), a true feasibility study that confirms floors can be added to an existing building still requires:

  • Enough time (a couple of months)
  • Considerable zoning and building code analysis
  • Review of plans, if available
  • Destructive and invasive investigation if the plans are not available

Let’s take a closer look at some of these factors that must be vetted out:


I. Entitlements – Zoning and Land Use

A land use attorney is often the best professional to assess the zoning implications. He or she will thoroughly review a series of entitlement issues, as they relate to vertical expansion:

  • Is the use changing?
  • Is the proposed use allowed in the zone?
  • Is the building historically protected?
  • Is there a neighborhood committee that will have input and approval rights?
  • Does the proposed addition exceed the allowable height and/or number of floors in the zone?
  • Does the proposed addition exceed the allowable FAR (Floor Area Ratio) for the site?
  • Does the increased FAR create a need for a traffic study, if applicable?
  • Will the addition cause new shadows on existing properties in areas where shadow studies are required?

Obtaining a waiver or variance from the local zoning authority is often very difficult or impossible. Furthermore, variances are not granted due to financial hardship.


II. Building Code – Non-Structural

Even if the proposed addition of a floor or two is allowable per the zoning code and applicable land use restrictions, there is still a set of building code requirements specific for non-structural components.

These include:

  • Height and area restrictions of the building coded for the type of construction (this is in addition to the height and area restrictions of the zoning code)


1102.1 Height Limitations- An addition shall not increase the height of an existing building beyond that permitted under the applicable provisions of Chapter 5 of the International Building Code for new buildings.

1102.2 Area Limitations- An addition shall not increase the area of an existing building beyond that permitted under the applicable provisions of Chapter 6 of the International Building Code for new buildings unless fire separation as required by the International Building Code is provided.

Exception: In-filling of floor openings and nonoccupiable appendages such as elevator and exit stairway shafts shall be permitted beyond that permitted by the International Building Code.

1102.3 Fire Protection Systems- Existing fire areas increased by the addition shall comply with Chapter 9 of the International Building Code.

  • Additional code requirements and potentially costly upgrades (such as fire protection and accessibility) triggered by the cost of upgrading the building with new floors or stories
  • Additional upgrades and expenses to fire protection systems (sprinklers, stairwell ventilation, fire alarms, fire rated assemblies, etc.) may occur if, as a result of the vertical expansion, the building will be newly classified as a high rise (over 70-75 feet in many jurisdictions)

Further considerations include:

  • Existing Means of Egress: are there enough? Are their capacities (width of egress stairs and doors) sufficient?
  • Existing Elevators: what impact will the additional floors have on the elevators? Would adding new floors now require the addition of an elevator if none is present?
  • Utilities: are the gas service lines, electrical service and switchboard, sprinkler lines and water lines of sufficient size and capacity to support the added floor area? Is a fire pump required? Is the existing domestic water booster pump, or gas booster adequate? Do these items need to be added in order to proceed with the vertical expansion?


III. Structural Considerations

There are several categories of structural implications to consider:

1. The existing structure’s capacity to carry added weight from the new floors' dead loads (construction materials) and live loads (people or snow and other non-permanent loads)


Any existing gravity load-carrying structural element for which an addition and its related alterations cause an increase in design dead, live or snow load, including snow drift effects, of more than 5% shall be replaced or altered as needed to carry the gravity loads required by the International Building Code for new structures. Any existing gravity load-carrying structural element whose gravity load-carrying capacity is decreased as part of the addition and its related alterations shall be considered to be an altered element subject to the requirements of Section 806.2. Any existing element that will form part of the lateral load path for any part of the addition shall be considered to be an existing lateral load-carrying structural element subject to the requirements of Section 1103.3.

Exception: Buildings of Group R occupancy with not more than 5 dwelling units or sleeping units used solely for residential purposes where the existing building and the addition together comply with conventional lightframe construction methods of the International Building Code or the provisions of the International Residential Code.

  • The existing roof capacity to support the loads required for an occupied floor - without reconstruction or major rehabilitation
  • The existing columns’ capacity to support the new floors and roof dead and live loads, without reinforcement
  • The existing footing or foundation system’s capacity to support the additional loads
  • The feasibility of adding new columns or a new independent structure to support the
    additional floors

This assessment becomes much more difficult if no existing building plans are available, as it will then require destructive investigation to determine:

  • Sizes of beams
  • Columns and footings
  • The reverse engineering of structural capacities

Without building plans, the finishes of several columns will need to be removed in order to determine the sizes of the columns for the full height of the building. At a minimum, one exterior and one interior column should be exposed.

The same applies to the existing roof framing that will be converted to floor framing; the sizes or all members will need to be determined.

Finally, footing or foundation components will need to be exposed by excavating inside and outside the building to determine sizes.

2. The lateral loads imposed on the structure by vertical expansion

Lateral loads include both wind and seismic loads.

Seismic loads are concerns in many major marketplaces in the United States - not just obvious locations such as the west coast, Hawaii, or Alaska. Cities like Boston, New York, and Philadelphia, for instance, all fall into Seismic Zone 2A.

Seismic upgrades to existing structures can prove to be very expensive and disruptive, particularly for buildings designed and built prior to modern day building codes.

Unreinforced masonry construction is especially troublesome to reinforce.

Chapter 11 of the International Existing Building Code (IEBC), which has been adopted by all states, requires that an existing building undergoing a vertical expansion be designed to withstand reduced seismic loads from the current code if the vertical expansion causes an increase in the story shear at any level by more than 10%.

The story shear is the design lateral force due to seismic loads at any one story, or floor, of the building. The load is a portion of the weight of the structure, and it’s impacted by the height above grade.

Designing a lightweight structure for the vertical expansion that will not exceed the aforementioned 10% increase may prove difficult, or costly. An exception would be a very heavy existing building, like a cast in place concrete structure, for example.


Where the addition is structurally independent of the existing structure, existing lateral load-carrying structural elements shall be permitted to remain unaltered. Where the addition is not structurally independent of the existing structure, the existing structure and its addition acting together as a single structure shall meet the requirements of Sections 1609 and 1613 of the International Building Code using full seismic forces.


  1. Buildings of Group R occupancy with not more than five dwelling or sleeping units used solely for residential purposes where the existing building and the addition comply with the conventional light-frame construction methods of the IBC or the provisions of the International Residential Code.
  2. Any existing lateral load-carrying structural element whose demand-capacity ratio with the addition considered is not more than 10% greater than its demand-capacity ratio with the addition ignored shall be permitted to remain unaltered. For purposes of calculating demand-capacity ratios, the demand shall consider applicable load combinations with design lateral loads of forced in accordance with Sections 1609 and 1613 of the IBC. For purposes of this exception, comparisons of demand-capacity rations and calculation of design lateral loads, forces and capacities shall account for the cumulative effects of additions and alterations since original construction.

3. The impact of vertical expansion on the structure of adjacent buildings

I find this is often overlooked, even by the most thorough of engineers.

If the abutting building is the same height as the subject building, the addition of even one story can impose snow drift loading on the adjacent structure.

Of course, this consideration only applies to properties affected by a northern US climate. The legalities of how this may be addressed are beyond the scope of this article, but, needless to say, the implications can be expensive and extensive.

As you can now see, asserting viability for vertical expansion is far more complex than, say, determining the capacity of a roof structure, or even the columns and footings of a building.

If vertical expansion is the make or break component of your next property acquisition deal, be sure to allocate sufficient time and resources.

They will be indispensable to ensure, with a reassuring level of confidence, that the expansion is not only possible, but also that it can be achieved within a reasonable time frame and budget.



There are far greater implications to adding a new story to an existing building than most investors realize:

  • A land use attorney would need to uncover entitlement and zoning implications
  • Building codes for non-structural components must be accounted for
  • Building plans must be evaluated to confirm roof, columns, and foundation have capacity to hold added weight of new floors (or extensive investigation must be done in lieu of building plans)
  • Ensuring the story shear is not increased by more than 10% as a result of each added floor

If vertical expansion is the make or break component of your next property acquisition deal, be sure to allocate sufficient time and resources.


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