Hi Toby - not a lot to go on here.
Age of property, method of construction, level of insulation, ventilation method etc are all factors to consider as outlined below.
- Ventilation of spaces and insterstitial spaces - spaces within the building's construction)
- The level of relative humidity - the amount of water vapour in the air
- Temperature of the building envelope - how warm at the surfaces the moist air will reach
- Thermal bridging - specific locations where the building envelope is conducting heat strongly.
This is what one considers when dealing with condensation arising from within the building.
To this can be added
- Leaking service pipes
- Water ingress from outside
- Groundwater levels
- Flooding.
These are instances of water coming from outside the house.
Until these facts have been assessed I would advise that you don't spend money on a dehumidifier, as other issues may be causing the condensation.
ONQ
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.