Ok i know about the legislation about fire extinguishers 

For any story in a building with a floor area equal to or less than 400 m² there should be

a minimum of 2 x class 'A' extinguishers with a minimum combined fire rating of 26A.

 

examples:

floor area 130m² ..........  2 x 13A extinguishers

floor area 390m² ..........  2 x 13A extinguishers

 

But why do I see people with one 13a fire extinguisher and a co2 shouldn't they have two 13a fire extinguishers and a co2. Are these other companies not been told or am i missing something 

 

Property i visit is less than 400m2 

Because some people follow common sense (& risk assessment) and do not rigidly follow the recommendations of BS5306-8 which has been ruined by the trade representatives on the Committee to sell extinguishers.

If it's a floor area under 400 sq.m. then in some circumstances the potential 4 extinguishers per floor (if you use separate CO2 for electrical risks) is just plain overkill, especially in premises small enough for a single exit.

Yeah i understand having 2 water and one co2 could be over kill but if i recommend 1 water 1 co2 and not follow the legislation what implications do i put my self in? 

As Anthony said, the British Standard is a recommendation, so a client can risk assess fire extinguishers out of a premises. As an engineer, all you can do is follow the British Standard and make recommendations based on that. If the client decides that they do not require the quantity you have recommended, that is their choice and as long as you note it on your paperwork and the client signs to say that they fully understand they are going against the Standards, you have fulfilled your duty.

While I agree that 2 x A rated with a combined 26A would be massively overkill in a 5m square premises with a single exit, there is always the possibility of an extinguisher failing. It is unlikely, but possible. If a single extinguisher is used to cover a floor, no matter how small that floor, and it fails then there is a serious problem for anyone trapped on that floor.

Saving a few £££ on an extinguisher isn't worth risking a life.

Extinguishers are not provided for means of escape, that's an old myth, but to mitigate the effects of fire and if one fails you get out. The risk of failure is minimal if you have a suitable system of maintenance and buy decent quality kit.