Foundation Area and Maximum Number Of Storeys
Posted by Andrew Bairstow on 08-Jul-22 05:42 PM
Foundation Area

A new term 'Foundation Area' has been introduced with the 2021 NCM. It is defined as:

Foundation Area = Total conditioned Area / Nstoreys

DesignBuilder v7.1 calculates the Foundation Area automatically using this formula and uses the input value of Maximum number of storeys to do this. The Maximum number of storeys parameter has existed in SBEM since the 2010 NCM was introduced.

Maximum Number Of Storeys

Is defined in the iSBEM Compliance Guide as: Number of storeys of the building being modelled. If the building does not have the same number of storeys all over the floor plan, input the maximum number of storeys.


Whereas the 2021 NCM defines Nstoreys as: the total number of storeys in the entire structure that encases the modelled building, including partial storeys and storeys containing dwellings and buildings other than dwellings (whether or not they are part of the building being assessed, i.e., represented in the software model)

In most models these two terms (Nstories and Maximum number of storeys) will be the same and in these cases, DesignBuilder's automatic calculation for Foundation Area will be correct. However there will be assessments in which the two parameters will be different and for such a situation the Foundation Area can be overridden. Examples of situations which require some manual calculation of Foundation Area include office blocks where only one storey is being modelled and sites comprising multiple buildings of different heights. DesignBuilder cannot calculate a parameter which is based on data that is not contained in the model and so for these situations it will be necessary to manually override the calculation. It is considered that defaulting this calculation to be based on the existing Maximum number of storeys input should provide an accurate value for Foundation Area in the majority of circumstances and is preferable to using the only realistic default of 1.

Example 1

Multi-storey block where individual floors are modelled. A five storey block: only 2 floors are assessed.

The Maximum number of storeys is input as 2:

Foundation Area is calculated based on Nstoreys=2

The TER and TPER are calculated based on this Foundation Area:

Foundation Area has been calculated based on Nstoreys = 2. However, whilst the Maximum number of storeys should be 2, Nstoreys should be 5. Changing the Maximum number of storeys will affect the Heating/Cooling calculation so that must not be edited. 

Thus the Foundation Area can be recalculated based on the value of Nstoreys (2) used to derive Foundation Area (400) along with the correct value of Nstoreys (5).

400 * 2 / 5 = 160

This can now be input manually:

The TER is now calculated correctly

See also: XML will not lodge due to Negative TER