There is no specific legal definition of this term (unless the deed itself contains one; if it does, that definition will control). When there is no specific legal definition, the law looks to the plain or customary understanding of the term in ordinary usage, which would mean that to "begin construction" is to start the actual work of contruction--basically, putting a shovel in the ground and starting to did the foundation, etc. Paperwork (like a permit) or planning (like drawing up blueprints, contracting with a builder, etc.) would not qualify--it is actually starting to build that would be the "begin[ning] [of] construction."