Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
Section B. Establishment of non-defended localities
The UK LOAC Manual (2004) states:
5.38.1. The term “non-defended locality” has a special meaning. It is one where all the following conditions are met:
a. all combatants, as well as mobile weapons and mobile military equipment must have been evacuated;
b. no hostile use shall be made of fixed military installations or establishments;
c. no acts of hostility shall be committed by the authorities or by the population; and
d. no activities in support of military operations shall be undertaken.
5.38.2. This rule is an extension of the rule on undefended localities but depends on more formal arrangements. Where opposing ground forces are in contact, a commander might decide to withdraw from an inhabited area and allow the enemy to occupy it to avoid bloodshed among the civilian population or to preserve important historical or cultural sites. He can declare the place a non-defended locality.
5.38.3. The declaration of a non-defended locality should define as precisely as possible the limits of that locality and should be addressed to the adverse party who should acknowledge its receipt and then treat the locality as non-defended unless any of the conditions mentioned above is not fulfilled. In that event, it must inform the party making the declaration. It follows that a non-defended locality can be created without express agreement between the parties. It remains a non-defended locality until the party making the declaration withdraws it. Although no procedure for withdrawal is laid down, it should not take effect until notice of withdrawal has been given to the opposing party.
5.38.4. Even if all the conditions are not met, the parties may agree between themselves to treat an area as a non-defended locality. The agreement should be in writing and should specify the exact geographical limits of the locality, the date and time of the entry into force of the agreement and its duration, rules on marking the locality and agreed signs, persons authorized to enter the locality, methods of supervision (if any), whether and under what conditions the locality may be occupied by enemy troops.
5.38.5. If, in any case, an area is to be treated as non-defended, the party controlling it is responsible for marking it with agreed signs, especially on its perimeter and on highways. Some ingenuity will be required to devise signs visible from aircraft. Distinctive radio or electronic signals may be needed instead.
5.38.6. Even if it loses its non-defended status, the locality will benefit from other protection available under international law. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.38.1.–5.38.6.
With regard to internal armed conflict, the manual provides that (in addition to the prohibition on attacks against undefended localities) “[t]he other rules on protective zones applicable in international armed conflicts may be applied by analogy to internal armed conflicts”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.25.2.