You need to look at The Regulatory Reform (Fire Safety) Order 2005 if you're in England or Wales and comply with the relevant section(s) regarding emergency routes and exits (means of escape) - in particular section 14:

Emergency routes and exits

14.—(1) Where necessary in order to safeguard the safety of relevant persons, the responsible person must ensure that routes to emergency exits from premises and the exits themselves are kept clear at all times.

(2) The following requirements must be complied with in respect of premises where necessary (whether due to the features of the premises, the activity carried on there, any hazard present or any other relevant circumstances) in order to safeguard the safety of relevant persons—

(a)emergency routes and exits must lead as directly as possible to a place of safety;

(b)in the event of danger, it must be possible for persons to evacuate the premises as quickly and as safely as possible;

(c)the number, distribution and dimensions of emergency routes and exits must be adequate having regard to the use, equipment and dimensions of the premises and the maximum number of persons who may be present there at any one time;

(d)emergency doors must open in the direction of escape;

(e)sliding or revolving doors must not be used for exits specifically intended as emergency exits;

(f)emergency doors must not be so locked or fastened that they cannot be easily and immediately opened by any person who may require to use them in an emergency;

(g)emergency routes and exits must be indicated by signs; and

(h)emergency routes and exits requiring illumination must be provided with emergency lighting of adequate intensity in the case of failure of their normal lighting.

How you choose to do this is up to you. You can follow industry guidance such as BS9999, the DCLG FRA guides, etc., or you can deviate from it, provided you can justify that the standards achieved fulfil the requirements of the legislation:

An excerpt from the DCLG FRA guide for offices and shops:

Your existing fire safety arrangements may not be the same as the recommendations used in this guide but, as long as you can demonstrate that they meet an equivalent standard of fire safety, they are likely to be acceptable. If you decide that your existing arrangements are not satisfactory there may be other ways to comply with fire safety law. This means there is no obligation to adopt any particular solution in this guide if you prefer to meet the relevant requirement in some other way.

So, to answer your question, yes you can deviate from what is considered best practice, but in this case (in my opinion) not protecting the external staircase would be quite a serious deviation, and as such would require robust mitigating factors to justify it. That's not to say it couldn't be done, but you most likely wont get any meaningful advice on how to achieve it without someone having seen the premises.

Edited by user 18 June 2019 16:24:39(UTC)  | Reason: Not specified