TITLE 21--FOOD AND DRUGS
CHAPTER I--FOOD AND DRUG ADMINISTRATION
DEPARTMENT OF HEALTH AND HUMAN SERVICES
SUBCHAPTER B--FOOD FOR HUMAN
Sec. 117.20 Plant and grounds.
(4) Be constructed in such a manner that floors, walls, and ceilings may be adequately cleaned and kept clean and kept in good repair;
Sec. 117.35 Sanitary operations.
© Pest control. Pests must not be allowed in any area of a food plant. Guard, guide, or pest-detecting dogs may be allowed in some areas of a plant if the presence of the dogs is unlikely to result in contamination of food, food-contact surfaces, or food-packaging materials. Effective measures must be taken to exclude pests from the manufacturing, processing, packing, and holding areas and to protect against the contamination of food on the premises by pests. The use of pesticides to control pests in the plant is permitted only under precautions and restrictions that will protect against the contamination of food, food-contact surfaces, and food-packaging materials.
Would PROBABLY be what your FDA inspector would reference. For pest control you need to be able to walk the walls for inspection (to ensure no pests have exploited a means of ingress). If you can't walk the interior, (some of us fat folks need more then 18 inches) then you can't (unless you can reasonably prove otherwise) adhere to that FDA standard.