BS EN752 : 2017 Drain and sewer systems outside buildings. Sewer system management states
Manhole steps should not be used for access / egress in manholes deeper than 3.0m from cover level to the soffit of the pipe.
Manhole steps should be Type D Class I double steps conforming to BS EN13101 with a minimum tread width of 320mm.
Not less than 150mm foot room should be provided between the steps and the chamber wall behind them, when measured at the centre of the step.
Not less than 900mm of clear space should be provided on any landing between the face of the steps and any wall or handrail. This means that the minimum chamber size should be 150mm + 900mm = 1050mm.
Note that Sewers for Adoption requires that the minimum nominal internal dimension of a manhole is 1200mm. Therefore, a 1050mm chamber would not meet the requirements of Sewers for Adoption.
BS EN752 states
Fixed, corrosion resistant ladders conforming to BS EN14396 (or stairs) may be used in place of steps and should be used for depths over 3.0m.
Not less than 200mm foot room should be provided between the rungs of ladders and the chamber wall behind them, when measured from the centre of the step.
Stringers should be adequately supported from the manhole wall at intervals of not more than 3m.
Stringer supports should not be built into the wall but bolted to built-in cleats to facilitate renewal.
Steps and Ladders
With further reference to BS EN752, where ladders or steps are in use, a means of fall prevention should also be provided. In order to deter unauthorized access, some manholes are not fitted with permanent ladders or steps. In such cases, personnel are normally winched into and out of the manhole, however, an alternative means of rapidly removing personal including casualties from the manhole, should provided for use in an emergency.