I always thought the nominal (on-paper) value of a property was decided by the valuation by an Estate Agent and confirmation by acceptance by the solicitor who does the conveyancing, at the point at which the property changes hands. An Estate Agent will mariegive you a free valuation.
Your home (principle private residence PPR) was inherited from parents and is co-owned with a sibling so you do not have first-time buyer status if that was an implicit question. You already own a property so the cottage is an investment property........if that was an implicit question. In terms of how your purchase and ownership are categoriesed it doesn't make any difference whether you are renting it out or keeping it shuttered when you yourself are not using it.....if that was an implicit question.