Yes, they're nice to have, and I am sure they have their perks, but to me it seems like more of an aesthetic preference - one that generally isn't a deal-breaker.Or you could view it as an insurance investment.Now, if I had a tenant who happened to be a carpenter and who wanted to build a storage shed in his yard, I would offer to pay for the supplies because I could market that to future tenants and hike up the rent. 

You could likely get a variance, but even then would be be shoe horning a house between the other two. 

Possibly think about renting it out as storage/ install some coin operated laundry machines.

There's potential to add paid laundry or extra storage facilities in the unfinished basement - which might increase your returns a bit.

I thought that due to the rise in e-commerce, we could buy these spaces and convert them into "last mile" storage facilities for online retailers.

given the current situation - Self storage (couple options) - Dialysis centers (couple sets of dialysis centers)I would estimate that this half of the investment would generate a 8-11% IRR long term.

Same is true with outside water spigots, garbage disposals, storm doors, storage sheds, and garage door openers. 

A friend of mine just approached me with a deal on a self-storage business.

Or is the single family one parcel, duplex is another, maybe the garages are on a third (although not sure what value these have unless you plan on renting as self storage)

Really well designed spaces with great storage and flow - small but comfy outside space - etc.