Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 42. Works and Installations Containing Dangerous Forces
Section B. Placement of military objectives near works and installations containing dangerous forces
The UK LOAC Manual (2004) states:
To avoid the risks to the civilian population, there is an obligation on the parties to the conflict “to endeavour to avoid locating any military objectives in the vicinity of the works or installations” containing dangerous forces. They may install weapons such as anti-aircraft or anti-missile missiles solely for the defence of these installations and those weapons are also protected from attack so long as they are not used for offensive purposes. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.30.7.