Should you use Perez' permission or Perez's permission in a letter?

Answer 1

The correct singular possessive form is Perez's.

However, some people treat a singular noun ending in 'z' as they would a singular noun ending in 's'. There are two accepted possessive forms for singular nouns ending in 's':

Add an apostrophe (') after the existing s at the end of the word: Paris'

Add an apostrophe s ('s) after the existing s at the end of the word: Paris's

So, if you wish, you may use the possessive form Perez'.

Answer 2

Both forms, Perez' permission and Perez's permission, are correct. The choice between them depends on the style guide you follow or personal preference. Just be consistent in your usage throughout the letter.