tl;dr all Life Hacks are DIY solutions, but not all DIY solutions (or questions) are Life Hacks.
So are lifehacks just DIY solutions, or is there a difference?
There damn well better be! This is the single most important discussion here - if we can't come to an understanding of what separates a question asking for a "hack" and a question asking for a HOWTO, then we're done - close it up & go home.
Many existing sites have some amount of overlap; Home Improvement, Gardening & Landscaping and Seasoned Advice all touch on a few of the same topics, albeit from somewhat different perspectives. This site is not like those sites however. As Robert wrote in regards to a related question,
If you're going to ask how to cover a nail hole effectively, you had better specify why spackle will not work for you. If you're simply asking how to lose weight, or change a tire; that's not a life hack. Boiling water is not a lifehack. Cooking food is not a lifehack — not unless there's something so novel or so unique about the circumstances, that the solution isn't really well-known or already commonly taught in that subject space.
We cannot define this site in terms of what it isn't about, so we desperately need to settle on a definition for what it is about. A reasonable model to follow here is the existing site Sustainable Living:
Sustainable Living Stack Exchange is for folks dedicated to a lifestyle that can be maintained indefinitely without depleting available resources.
Most of the topics asked about on that site would be perfectly appropriate on other sites, and most of the answers would be just fine elsewhere as well. However, the reverse is not true! Plenty of gardening questions, for instance, are not specifically concerned with sustainability - they're things that any gardener might need to know. Same for cooking questions, DIY, etc. The ones appropriate for Sustainable Living are those having a "direct relation with sustainability" - that is, they either implicitly or explicitly express a desire for a sustainable solution.
I see the same principles applying here. Regardless of whether a question's topic would be accepted elsewhere, a question asked here must fulfill two requirements:
It must be a clear, specific question on a practical topic. The asker must communicate specifically what he's looking to accomplish, what the constraints are, what research he's already done, etc. - in other words, all the usual advice for asking a good question applies first - if it's not a good question, it doesn't matter if it's on-topic or not (and in fact it may be impossible to determine).
It must implicitly demand an unconventional solution. What, specifically, "unconventional" means is up for debate, but it's pretty obvious when someone isn't looking for one. Fixing floors and household appliances are ordinary HOWTO tasks; indeed, there are professions dedicated to them. Whether or not there exists a Stack Exchange site for professionals in these fields, simply learning a new skill is not a Life Hack.
Now, here's the punchline: if a question fully satisfies #1, we can often just assume that it satisfies #2 as well! Unless something in the question prohibits unconventional answers, we can just go right ahead and provide them; if it turns out the asker forgot what site he was on and wanted a conventional solution, then he's free to go ask somewhere else.
But... We must guard against the temptation to provide conventional solutions to ambiguous questions. If the solution we provide for squeaking floors is "attach the boards firmly to the joists" then we're not really fulfilling the mission of the site - we're just duplicating common DIY knowledge for no good reason.