I had to sell my parent's house last year. The hardest part was clearing it to a level where I could make it look presentable enough to sell, it took me nearly six months.
One of the most important things is to make sure you find all their documentation and file it for later use. In my case I went through every box full of paper and had two bin bags and two small boxes next to me! One for for rubbish, one for recycling (tons of junk mail and catalogues mainly), a box for memorabilia like personal letters and cards, and a box file for all bills and bank statements. I designated one room in the house for stuff for charity and ended up giving five van loads to Age Concern as they had a driver who would come to collect it. My parents never threw anything away........
As I went along, I filed all the financial bumph I found and ended up with huge files and many file dividers for each of them. It was worth it because then I could start the long process of sending my LPAs off to gas, elec, insurance companies, and all services before I actually got an offer on the house. If you can get multiple copies of your Power of Attorney documents it will save you a lot of time, as nearly every company wants to see a certified copy.
My parents savings were scattered all over the place, I think financial advisors must have told them to spread their risk, and in some cases all I found was one very very old statement. The whole process is like a convoluted treasure hunt that you never asked to join
Once I got an offer for the house, I got loads of questions about boundaries and meter locations, but I was just honest with them and said I never lived in that house, so there were many things I could only guess at. I was glad I'd saved all receipts for window replacements and any extra bits added to their house, as there are questions about conservatories and boiler servicing etc on the questionnaires you have to complete before exchange of contracts.
It was a very sad thing to do, as my father was devastated at having to leave his home, and his dementia made him paranoid that people were stealing his stuff and at one stage he even accused me of selling their house for my own financial gain, very hurtful. We ended up keeping boxes and boxes of their favourite possessions and making a memory shed in our garden for my parents to visit, which made it slightly easier than having to chuck the lot away. Be warned, you will probably find an awful lot of personal secrets that you wish you hadn't seen
Good luck, I hope your parents were tidier and more organised than mine x
