Grammar JLPT Level N4 - Lesson 23

Like が必要(ひつよう), 必要(ひつよう)がある is an expression which is used to highlight the 'need' or 'necessity' of something. The primary difference being that 必要(ひつよう)がある highlights the need to 'do (A)', and will therefore be used with verbs (without the need for nominalization).
To use 必要(ひつよう)がある, just attach it after the plain form of the verb that you want to highlight the 'need' for.

• 明日(あした)にテストがあるから、勉強(べんきょう)をする必要(ひつよう)がある。
  Because I have a test tomorrow, it is necessary to study.
• 息子(むすこ)はまだ小(ちい)さいので、いつも一緒(いっしょ)にいる必要(ひつよう)がある。
  Since my son is still little, it is necessary for me to be with him at all times.

As we can see, this construction is simply the combination of a verb + 必要(ひつよう) (creating a verb modified noun), the case marking particle が, and the う-Verb ある. In this way, it just means 'the need of doing (A) exists'.
Naturally, this grammar pattern may also be used negatively, to highlight that there is 'no need to do (A)', or that 'doing (A) is not necessary'.

• カイルは日本語(にほんご)が分(わ)かるので、書類(しょるい)を英語(えいご)に訳(やく)す必要(ひつよう)がない。
  Since Kyle understands Japanese, there is no need to translate the documents into English.
• 娘(むすめ)は元気(げんき)になったので、病院(びょういん)に行(い)く必要(ひつよう)がありません。
  Since my daughter got well, there is no need to go to the hospital.