35 Penalty for contravening building regulationsIs it illegal... ?
As far as I know it is not a criminal offence to not follow building regulations in your own property.
If a person contravenes any provision contained in building regulations, other than a provision designated in the regulations as one to which this section does not apply, he is liable on summary conviction to a fine not exceeding level 5 on the standard scale and to a further fine not exceeding £50 for each day on which the default continues after he is convicted.