Flues must be at least one metre above the highest part of the nearby buildings. In many circumstances it will need to be significantly higher.
The appropriate stack height should be determined using dispersion modelling, and not only by using the Memorandum on Chimney Heights or the Technical Guidance Note (Dispersion) (known as a D1 calculation). These are not suitable when considering annual mean NO2 concentrations.
Flues should not be ‘hidden’ behind structures likely to affect the spread of emissions.