Alarm spell with metamagic extend every day, or spring for a scroll of permanency for it.
Check out the old 3.5 adventure called "Something's Cooking". It can be found online by simply searching for the name and 3.5.
I ran this adventure and it was probably my most fun experience ever in DnD/Pathfinder.
The wizard there has animated objects, a fire elemental (in the cooker) and a golem made out of pastry (or something like that)
Was very fun and could be adapted.
At 5th level, the wizard doesn't have much to protect aside from his spellbook and maybe a couple magic items.
What he needs is a good safe that is also lead-lined so it's detection proof. He needs to put this somewhere where lead won't look out of place in case someone tries to detect that. I'd suggest a built-in china cupboard with a lot of pewter dishes. Don't have Arcane Lock on the secret door as that's a red flag for thieves with any detection ability. Instead, make the unlocking mechanism something that can only be easily manipulated by Mage Hand, the catches far inside cracks.
He also needs to put out a dummy spellbook that would ideally be an old traveling spellbook of his. Chain it to a desk that's bolted down, and put a few other books with it that you've cast Secret Page on with the secret pages leading to Explosive Runes for whoever decodes the password. Put Arcane Mark and Arcane Lock on all the books for good measure.
If you're going traveling and are going to be leaving your great book and assorted valuables unguarded except by your safe, invest in a bottle of Sovereign Glue and a flask of Universal Solvent. Gum up the works of your safe with the Sovereign Glue before you leave. Unstick it with the Universal Solvent when you get back. Short of someone taking the china cupboard and the wall with it, they're not getting your stuff.
At 5th level, the wizard doesn't have much to protect aside from his spellbook and maybe a couple magic items.
Where is the wizard, who's the threat and what the state of mind.
In a city or a village or out in the whops?
Are thieves out for the family silver, is it another mage or the local mafia?
How much paranoia is involved, it pays to be careful or is every out to get me?
Hire someone to check up on the place every few days, something like a cleaning lady or just someone to feed the dog. Become friendly with the local watch, offer them magic services at a discount. And get your party's thief to create some non-lethal traps.
If you are willing to go without him, leave your familiar at home to look after things.
If he is solely looking to protect his spellbook he can have a basement under his basement that is accessible only through Gaseous Form. This "sub basement" could have his master spellbook and such. make the "small tunnel" to access it, lead from the house not from the primary basement. (small crack behind the oven or behind a bookcase would be very difficult to find but relatively easy to access while in gaseous form).
Bad part: this means casting the spell twice to fetch his spell book which means all or 2/3 of his max spell level for the day.
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Wallpaper the room with sheets of Explosive Runes scrolls.
Leave a note on the front door with Illusory Script spell, make the Suggestion if someone reads it to be "Leave this cottage alone - there is nothing of interest here." Check back in 5 days to renew.
Put a Continual Flame atop the front door so the outdoor light is always on (and folks can read the Illusory Script).
Phantom Trap on the front door lock or another access point.
A Magic Mouth inside that says "The Master has been alerted to your intrusion. He is on his way..."
Arcane Lock every portal.
Hire some strongarms to protect the joint. At a few silver a day with a bonus at the end if the place is not bothered, your 2500gp goes a long way.