Here's the breakdown:
* Permit: This is the base verb, meaning to allow or authorize something.
* Permitting: This is the present participle, which is used to form the continuous tenses (e.g., "They are permitting the construction.") and sometimes as an adjective (e.g., "permitting conditions").
The double t in "permitting" follows the standard rule for forming the present participle of verbs ending in a single consonant preceded by a single vowel: you double the consonant before adding "-ing."