I just checked in, and wanted to thank all of you for some straight-talking and very sensible advice. I'm particularly grateful to those who have shared their own experiences, as this has raised several new issues I hadn't even thought about. And yes, DoJo is right - I was thinking of the book as a way of opening a conversation about the amount of 'stuff' they have, not as a solution!!
To answer the questions: yes, the bungalow next to GFIL's old one is on the market. And it's MIL rather than FIL who I think is the driving force behind this idea. They are both hoarders in different ways. MIL can't bear to get rid of furniture (anything old is an 'antique') and knickknacks. FIL can't bear to part with ancient paperbacks or photographs (there are literally hundreds of thousands of these, none of which will ever be looked at again). I had thought about encouraging them to do up the bungalow and move in, but I do fear that having both houses will just double the clutter!
I would be more than happy to help them to declutter. In fact, I rather enjoy tidying and organising, which is part of the reason I was looking for other opinions from those who are less minimalist in their approach! I itch to reorder things when I am there. There are three issues with this, however.
The first is that we live 5 hours' drive away.
That wouldn't be a problem in itself, but the second issue is that it takes my PIL a very, very long time to do even small things and to become extremely anxious and angry about being rushed. As postchildren perceptively notes, they are very 'old' for their age. I think they've always been old-fashioned, and this has been exacerbated by the fact that neither has worked since their 40s. They seem to have lost the capacity to make decisions or to execute them in anything like reasonable time. It takes literally hours just to get the out of the house. There's a very odd loss of lateral thinking too about the most efficient way to do a task too - for example, they will think that they have to get absolutely everything out of a room to move something at the back, rather than simply lifting it over other things. It's quite extreme - a job that would take DH and me about 5 minutes will take them well over a day. 10% of the delay is mobility issues, 90% is faffing. So I don't think we could be there for long enough to have a real impact - it would take literally weeks to get them to go through one room and frankly we don't have that kind of time because of work. There are also odd issues about only parting with stuff for money - they want to sell anything they get rid of, even though they are very wealthy, which adds a layer of complication to sorting things out. I can't work out whether this is an excuse, or whether they are searching for impediments to getting anything done!
The third issue is that they are incredibly controlling, even verging on bullying. MIL is utterly, completely bossy, so it would be a real role reversal for me to be helping her to organise things - she prefers to tell us how we should be living, and it can truly be a barrage of advice.
They don't really interact with anyone, including their own kids, at an emotional level - it's all very formal. I don't know how she would cope with the greater levels of equality implicit in the idea of my helping her to clear things. I am willing to try, though! (And to be patient trying).
I think the idea of telling them that we will just be junking a lot of their stuff when they're gone is actually quite a good one. It would definitely shock them and it would disturb the idea that the ancient rickety bedstead is actually valuable and not just junk. I also wonder whether one tactic might be to get them to buy brand new furniture as a 'replacement' for the older stuff. They have a (hilariously incongruous) love of Swedish design. This is high risk, though! What if we just double the amount of stuff again?!?!?