In UK projects, at least one external wall is required because it is necessary for an MCS compliant heat loss calculation.
External walls are part of the building’s thermal envelope and tell the system where heat is being lost to the outside. They also provide the connection to the outside design temperature, which is essential for accurate project calculations.
In addition, some related building element values, such as floor U value logic, depend on the presence of an external wall in the project.
Without at least one external wall, the project is technically incomplete and cannot be treated as fully valid for MCS compliant outputs.
In short: no external wall means the system cannot fully and correctly assess the building fabric, heat loss, and temperature conditions required for a compliant UK project.