tl;dr: its okay to work in barricaded garage but ONLY at night.
well, since i’ve been in exacly this situation i can tell you what happened.
1st: my toon had light step, bow as weapon, integrated toolset and full night vision + infravision mutations (so no light sources needed),
what happened: i found car in garage, brought over few components from other garages in city (this was done ~0000h) then i started to board the back window. all was good, rest of the night was spent taking the car part by part.
at morning, i did go quick trip to hunt for food, and then proceed with work. started new vechicle and began to install new stuff. but zombies came. hordes spawned outside garage, probably attraced by sound of me welding parts. few sniffed the door and break into. that was kinda fun fight, but in the end i ended with broken door and several corpses in the way. removing them and then bulding new door was easy, as no new horde spawned from THIS side of garage. i barricaded those back door also, and decided to postpone welding till night - as night seemd safe for work.
so i went to sleep only to be weaken by hulk that smashed my sleepy body into the floor. NOTE: all windows and door were still barricaded. what comes next is hardcore fight with hulk, in very closed space. note: wearing army helmet helps. when hulk was lying dead i noticed lots and lots of "?’ outside, probably zombies spawned thanks to hulk SMASH!'ing stuff inside. they were not attacking yet so i went to the smaller garage backroom, and barricaded myself (making door and boarding it) there. and went to sleep - no zombie come or spawned.
i woke by the night, and finished my shiny new turbo electric buggy with TWO (yay!) solars, by about 0900h, with well, horde that started to try to bash thier way into. filled battery, opened the front doors and saw… well… kinda big amount of zombies outside. thankfully with functioning vechicle i just rushed outside, ramming my way out of city.