I would not mind because apparently:
1. He has shown that he is a respectful guest
2. You have a large enough house that it doesn't feel cramped
3. Money for food, etc, isn't an issue if you have enough and he can pitch in.

Also, I happen to be fond of all my BILs, and I suppose you are too.

I would just look into whether you need to add him to your home insurance policy or whatever.

Is he hot?

I would let him. But he needs to do laundry, clean sheets etc. plus he should chip in on meals etc.

How many days a month are you talking?

Maybe you could have a Scott Pruitt type situation - he pays thirty bucks a night or something?

Yeah. There’s something off about a family member making 150k and being willing to offer zero financial contributions to their living situation. It’s rude of him to ask and offer nothing.

What happens if you want to host other guests? I would get resentful of this arrangement after a few months.

I used to have this job and I value privacy and organization so I kept an apartment. In reality it’s kind of a waste of money, so I can see why he would want to crash at your place. Maybe you can work out a mutually beneficial arrangement (he shares airline and hotel points? Pays you a nominal amount in “rent”? Babysits on weekends?) so you don’t feel like you’re being taken advantage of.

Did he offer anything like some money toward the wi-fi bill, electricity, water--? I can see how if it's truly a few days at a time that seems petty but if I were in his shoes I'd at least offer something toward utilities if I made this request. That would be considerate and demonstrate an awareness that he does more than sleep there. You don't have to take the money but I'd hope he'd at least offer out of courtesy.

In your shoes I would definitely tell him that his towels and sheets are his responsibility to wash. I'll wash a guest's linens of course but if he's a regular part of the household he'd need to wash his own linens. I'm assuming he doesn't expect you to wash his clothes!

What are the food arrangements? What do you do now when he stays with you? He won't be a guest if it's a regular arrangement. It might prevent any future misunderstandings if you, DH and BIL agree clearly on things like whether he gets his own fridge in the walk-out basement or he pays X dollars toward food or he buys all his own food and just has space in your fridge....Will he expect to cook? Do you think he'll expect to eat with the family at least sometimes?

I think the idea is fine but I also suggest you all work out these kinds of basic day to day things in advance. Little things like finding food you had counted on to serve for dinner is gone, or the washer is full of wet towels, etc., can sour an otherwise good arrangement, so to keep things good, it helps to be clear about expectations instead of making assumptions.

Oops. I see he is mid-30s. Freedom and flexibility obviously apply though!

And who knows...if you have kids one day he may be the cool uncle in the city where they want to live and it is his turn to put up your kids.

Let him. I was in the same position and it didn’t make sense to pay thousands each year when I only spent maybe 4 nights a month in my place. I was able to save a nice down payment for when I settled down and got a house.

Of course! Sounds like you like him well enough and your husband is on board. Do it for the year the re-evaluate. You look gracious, brothers reconnect, and your relationship may become special.

After a year if it has not worked then you can say why not....loved having you but not enough personal time for dh and I or gas bill increased significantly or need your space for au pair once baby arrives, etc.

How old is he? The younger workers want freedom and flexibility- not things or permanent roots. At least that’s what HR keeps telling us

OP here. I wanted to note his major belongings are in storage, I guess, so he wasn't proposing like moving in with all sorts of furniture, etc. He already has some suits and things in the basement closet we don't use.

Doesn’t sound like it would be a problem. I’d let him do it.

So does he not have a home anymore? No furniture at all? No “stuff”?

Sounds like it just doesn’t make sense for him to keep and pay for a place when he’s never there. Maybe he wants to make a big change but isn’t sure where yet. I’d do it for my BIL or sisters, especially if they were neat and quiet and never there. Not sure if you have kids but if so even better to have another adult in the house.

Yes he is mooching to save money. I wouldn't do this with my leech sister, but she is a mess.

My husband's brother is a consultant and travels for assignments or works remotely off his laptop. He apparently got rid of his condo near his company's HQ (no idea why) and asked us if he could crash with us this year (2019) for a few days at a time in-between assignments. He claims we have a perfect central location for his work assignments and airport preferences.

He did it a few times in 2018 and it was no problem – our house is big with a walk-out basement. He's single, makes good money (I assume $150K), he hardly drinks, he's a neat freak – he basically reads or works off his laptop. So it's not like he's some sketchy weirdo, but at the same time isn't this a really odd request? He is sort of cheap, so is this just mooching to save money? I don't know what would drive someone with his career to do this – single and $150K is a comfortable living. My husband is super easy going and doesn't mind at all.