House Valuation In Probate

[PATTJP]

Hi

My Grandfather & Grandmother signed the deeds of their house over to myself & my mother 50-50 a little over a 1.5 yrs ago. I now live in the house with my wife whilst my Grandparents moved to a smaller property. My Grandfather passed away recently and we are sorting out his estate. We know that this will not be exempt from his estate as it's not over 7 years but we would like to know how we agree a fair valuation on the house & when the valuation should be based from ie when we took over the house or at his time of death? Also we are planning major building work which will increase it's value by quite a fair amount. Who agrees the valuation, estate agents or inland revenue and then once agreed and any tax paid will that be the end of it or will we be liable if we come to sell in the future?

Also as the house was originally in both grandparents names would we be only liable for 50% of the value in this instance?

Thank you for any help anyone could supply.