If you read the 16 (or so) page deed of trust on your home, I am sure there are sections stating “you will preserve, maintain, and protect” the property. You may want to either consult your mortgage servicer, or perhaps a local attorney, before starting any project of this magnitude. While lender permission might not typically be sought for an addition or renovation, complete demolition and reconstruction of a house is a far more involved matter. You undoubtedly want a more qualified opinion that those on a mortgage Q&A site.
I agree with Ted that you need a more qualified opinion than what we can offer, however as an appraiser I can give you some advise (state of CA). If you completely tear down a home and in essence start from scratch (year built 2015), the city will treat your project as a new construction and your property taxes will sky rocket. If however you leave a section of the existing house (perhaps a single wall), remodel and add an addition you’ll be taxed on the improvements only. In my area its not unusual to see a $1,000,000 fixer with $750,000 of improvements completed post purchase. If all new construction (year built 2015) the property taxes (1%)will be based on the full market value (say $2,500,000 or $25,000 per year). If a portion of the original structure is kept than your tax rate will be based on the original purchase price $1,000,000 + improvements $750,000. By keeping a section of the original structure, the yearly property tax obligation has been reduced by $7,500. If you live there for the next 30 years, your total tax savings will be $250,000. Consult professionals in your area.