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Opening Up the Lot Line Windows

The Department of Buildings recently released Buildings Bulletin 2015-017 to clarify code requirements related to lot line windows on R-2 and R-3 buildings.

The baseline allowance for lot line windows has traditionally been a 10% permitted opening, but not for light and air.

lot line windows baseline allowance

The 10% opening allowance indicates that the area of the windows can total no more than 10% of the area of that story’s façade. Windows for light and air are required for rooms to be habitable (living rooms, bedrooms, studies, recreation rooms, kitchens and dining rooms). It’s from this baseline model that Buildings Bulletin 2015-017 permits greater options for openings and use of the rooms for light and air.

The allowances are contingent on three central factors:

Are the adjoining buildings on the same zoning lot?

If adjoining buildings are on the same zoning lot, the rooms with lot line windows may be habitable rooms. If they are not on the same zoning lot, the building owner may be interested in a zoning lot merger.

lot line windows same zoning lot

How far away are the adjoining buildings?

Lot line windows greater than 60 feet away from any neighboring building, measured in any direction, have greater window area allowances.




This allowance depends on whether the adjoining buildings are on the same zoning lot. If the adjoining buildings are not on the same zoning lot, lot line windows greater than 60 feet away from the adjoining buildings are allowed 100% openings, but not permitted for light and air. If adjoining buildings are on the same zoning lot, the DOB permits 100% openings but only 10% for light and air (the remaining 90% is non-operable glass).

Is There Hardship Meeting Light and Air Requirements?

Building owners who can demonstrate hardship meeting light and ventilation standards for habitable rooms and whose adjoining buildings are sprinklered or non-combustible are permitted a graduated allowance of openings.


In order for this permission to be granted, there are requirements relating to light, ventilation, air and window protection.

Assuming these requirements are met and adjacent buildings are on the same zoning lot:

  • The first 30 feet of lot line windows are permitted 10% openings for light and air.
  • The subsequent 30 feet are permitted the lesser of 45% openings permitted for light and air or the allowance in BC Table 704.8.
  • Lot line windows above 60 feet are permitted unlimited openings for light and air.

Buildings Bulletin 2015-017 offers flexibility for building owners with lot line windows based on three factors. First, consider the zoning lot situation with the adjoining buildings. Second, determine the distance of the lot line windows from the adjoining buildings. Finally, determine if there is hardship meeting light and air requirements and if adjoining buildings are sprinklered or made of non-combustible materials. Once these central factors are determined, buildings owners will have a full understanding of the opening and use allowances for their lot line windows.

Questions about lot windows? Have a project that needs code consultation or expediting? Contact us.