The lease does not prevent smoking, though rental tenants may find the flat owner has specified that the flat be non-smoking. Smoking on your balcony is discouraged, as it may cause a nuisance to your neighbours. Any cigarette ends thrown over your balcony will land on a neighbouring balcony or even on a car, and this will constitute a fire risk, as well as a nuisance. Such behaviour will be viewed as breaking your lease.