かたがた

'While also…', 'partly by way of…'. It marks one purpose of an action that also serves another purpose. It is used in polite greetings, visits and letters.

This is a formal, set expression found in polite speech and especially in letters and business correspondence. The main clause usually has a verb of visiting or writing, such as 伺う, 訪ねる or 申し上げる. It is stiffer than がてら, which is lighter and can also follow verb stems.

• 1.
  がてらN1「がてら」 has a light, casual tone with no polite coloring, and can also attach to a verb's ます-stem (e.g., 「遊びがてら」), with the second clause usually being a movement verb like 「行く」 or 「寄る」. For everyday actions like 「散歩」 the two are interchangeable, but for formal greetings use 「かたがた」.
• 2.
  ついでにN3「ついでに」 is neutral in register: it can attach to verbs in dictionary or た form, to 「noun + の」, or to action nouns like 「散歩」, and it can even stand alone as an adverb. 「かたがた」 attaches only to action nouns and sounds formal, so in everyday speech "while you're at it" is usually expressed with 「ついでに」.
• 3.
  かたわらN1「かたわら」 describes doing something else on a long-term basis alongside one's main job or primary activity; it attaches to dictionary-form verbs or 「noun + の」 and leans toward written style. 「かたがた」, by contrast, is about a single action that incidentally serves a second purpose and attaches only to nouns, so the two cannot be interchanged.
• 4.
  にかこつけてN2With 「にかこつけて」, the first element is merely a pretext and the real purpose lies in the second clause, often with a critical or mocking nuance. With 「かたがた」, both elements are genuine purposes accomplished in one stroke, with a formal tone and no negative connotation — the two are not interchangeable.

がてら also expresses a dual purpose but is less formal and can follow verb stems. ついでに marks a second action done incidentally, and かたわら means carrying on two activities in parallel over time, while にかこつけて means using something as a pretext.