As a competent DIY'er, am I legally allowed to install Hive myself?
The reason for the question is I have been asked to provide a cert as I'm selling the house.
If not, can someone point me in the direction of where it legally states I can.
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In the vast majority of jurisdictions, most DIY work is perfectly legal in a house you own/land contract, and live in. The only exceptions are things like Freon handling, gas lines, and several other crafts, due to the particular hazards (often indirect, e.g. Freon).
You're not allowed to work on houses you don't own for obvious reasons. In many crafts, including electrical, you're not allowed (or permit non-licensed people) to work on houses you rent out, because that would be a huge incentive for "slumlord repairs". Given that rental properties tend to already be at the "more distressed" end of the housing spectrum, this is a recipe for dead tenants. However it is presumed you have incentive to do good work on housing you expect to occupy yourself.
Further, certain minor jobs are always allowed (for homeowners not tenants) due to their simplicity, common-ness, and low chance of critical failure. For instance changing receptacles and switches (typically done for cosmetic reasons) even though this work is harder than it seems. I would certainly expect that changing thermostats on a 24VDC system would be on the short list.
That said, the "only work on houses you expect to live in" principle applies in spades. If you are gussying up a house for sale, STOP. Aside from the "no incentive to do safe work" factor, you're also locking them into choices they may not want. Don't lay white carpets (pretty but nobody wants to live in it), don't fit 20-space stuffed service panels, and don't pick a smart 'stat for them. Give them a price concession of a fraction of the upgrade cost and let them choose.
The final answer will depend on your specific jurisdiction. However, generally speaking
In most places, this means:
The actual specifics will vary depending on location. But as a general rule, you should be OK replacing your thermostat yourself.