What a DPC, DPM, cavity wall, safety glazing and room ventilation actually do, and why penetrations threaten them.
Damp-proof course and membrane
A damp-proof course (DPC) is a horizontal barrier built into a wall, typically at least 150 mm above outside ground level, that stops moisture rising up through the wall by capillary action. Piling soil, rubble or anything else against the wall above DPC level "bridges" it, giving damp a way round the barrier and up into the wall above.
A damp-proof membrane (DPM) does the same job in a solid ground floor: it is a continuous barrier, often polythene, that stops ground moisture rising through the slab, and it should link up with the wall's DPC so the two barriers form one continuous line of protection.
Cavity walls
A cavity wall is built as two separate leaves, an outer and an inner skin, with a gap between them; the cavity resists rain penetration and, with insulation added, improves thermal performance. If debris or mortar droppings are allowed to fall and lodge in the cavity, they can bridge it and carry moisture from the wet outer leaf across to the dry inner leaf, so the cavity must be kept clear.
Where a pipe passes through an external wall, sleeving and sealing the penetration protects the pipe and allows for movement, and keeps out draughts, water and pests, helping maintain the wall's weather resistance.
Safety glazing
Safety glazing, such as toughened or laminated glass, is required in "critical locations": places like doors and side panels, and glazing at low level, where people are most likely to walk into or fall against it. It limits the risk of serious cutting injury by breaking safely or resisting impact. Low-level glazing near where people work, such as beside a bath or basin, should normally be safety glass; unmarked ordinary glass there is worth flagging even if checking the glazing itself is not your job.
Ventilation
Air changes per hour (ACH) is a way of expressing the ventilation rate: it is the number of times the whole volume of air in a room is replaced with fresh air in one hour. In a bathroom or kitchen, an extract fan removes moisture-laden, polluted air at source, straight outside, rather than recirculating it, which reduces condensation and helps keep the air quality in the rest of the home. Ventilation in habitable rooms generally removes moisture, smells and indoor pollutants and brings in fresh air; combustion air for an open-flued appliance is a separate requirement again.