Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 9. Definition of Civilian Objects
The UK LOAC Pamphlet (1981) states: “‘Civilian objects’ are all objects which are not military objectives.” 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 4, p. 13, § 3(c).
The UK LOAC Manual (2004) states:
5.24.1. “Civilian objects are all objects which are not military objectives …”
5.24.2. The term “civilian objects” would normally include cities, towns, and villages as such but not military objectives within those places. It also includes foodstuffs and food producing areas, springs, wells, water works and other water sources, buildings and facilities used by civilians (so long as they do not fall within the definition of military objectives) such as housing estates and houses; apartment blocks and flats; factories and workshops producing goods of no military significance; offices, shops, markets and warehouses; farms and stables; schools, museums, places of worship and other similar buildings; and means of transport such as civil aircraft, cars, railway trains, trams, and buses. Special protection is also given to hospitals, internment and prisoner of war camps. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 5.24.1–2.
With regard to internal armed conflict, the manual states:
15.16.1. There is no definition of civilian objects nor is the term used in the treaties dealing with internal armed conflicts but the principles of military necessity and humanity require attacks to be limited to military objectives. Thus attacks on the following are prohibited unless they are being used for military purposes: civilian dwellings, shops, schools and other places of non-military business, places of recreation and worship, means of transportation, cultural property, hospitals and medical establishments and units.
15.16.2. UN General Assembly Resolution 2675, which was unanimously adopted and applies to all armed conflicts, can be regarded as evidence of state practice. Paragraph 5 of the resolution states: “dwellings and other installations that are used only by the civilian population should not be the object of military operations.” The principle of military necessity demands that civilian property may only be destroyed, or requisitioned for use, for necessary military purposes.
15.16.3. The old practices that tolerated during sieges the bombardment of civilian buildings (other than places of worship), hospitals, cultural property, indispensable objects and works containing dangerous forces, have not survived. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.16.1–3.