Answer: If a room falls outside the Ba-gua and is less than 50% of the width of the property, it is considered an "extension," and a remedy is not necessary. In Western Ba-gua Feng Shui, an extension is a PLUS as it expands the gua area it protrudes from... the opposite is true of a "missing" corner. However, I would not recommend a mirror for either situation.
The Ba-gua for an apartment, condo, house or commercial building never changes unless "major" reconstruction is done to the property. If a "remodel" is done in a different construction period than the original property, then a separate Ba-gua is required for the "addition." Before or after a remodel, you may want to call our office for a one hour phone consultation to review, remedy and/or enhance your property. Also, a phone or email "Yeah/Nay" consultation is recommended prior to purchasing "any" property.