Wizards that choose the abjuration school get the arcane ward ability, which absorbs a limited amount of damage for the player. The ward's hit points are replenished when the player casts abjuration spells. At level 4, deep gnomes get access to the svirfneblin magic feat. This allows them to cast nondetection (an abjuration spell) at will. Theoretically, a deep gnome abjurer could set aside some time to cast nondetection repeatedly, gaining up to 45 ward hit points.

The ward is once per long rest.

But it can be recharged when it loses HP. It can’t go over it’s initial HP value though. 

I was wondering why I remembered this ability being better when I read it that time. I overlooked the second paragraph.

So, yes you could burn an action to add 6 HP to the ward.

Dipping 2 levels of Warlock allows you a full recharge between fights with Armor of Shadows.

At 18th level you can also possibly do this with your free 1st or 2nd level spells.

Nondetection at will? Sounds great! Just be sure you don't need that ASI for something else, like Resilient (Con) or Warcaster.

If you have time, you can cast alarm as a ritual from level 1 on any abjurer.

Think about taking some levels in warlock, armor of Agathys and mage armor at will. this wouldn't require you to be a gnome or take an extra feat.

On the non-detection combo, the arcane ward specifically says that you must expended a spell slot to regain hit points to the ward form a spell.

Quote from Sniperwes20 >>

On the non-detection combo, the arcane ward specifically says that you must expended a spell slot to regain hit points to the ward form a spell.

 It does not specify that you must expend a spell slot. What makes you think that?

Sorry, I misread.