Victorian ground floor flat, RICS surveyor said there was rising damp. It had previously been damp proofed, there was mould at skirting-board level, which there could not be if there was rising damp.
Surveyor Tips:
- Not all damp at the base of a wall is caused by condensation, but it is much more common that rising damp by a factor of at least 10 X.
- It significantly cheaper to solve and will continue if the property has been treated for rising damp.
Root cause
Excess vapour causing condensation and mould.
Illustrations
The walls have been damp proofed against rising damp by injecting chemicals. Typically damp proofers replace ground floor internal plaster with an impermeable plaster, known as slurry. This increases condensation risk.
There is dampness to the front room, especially by the sockets.
Sockets are prone to dampness as a section of wall is cut out, a metal box installed and covered in absorbent plaster, which exacerbate the effects of condensation, see surveyor.tips/sockets. 
Looking closely at the sockets, there is a brown tea bag like discolouration, a sign of penetrating damp.
Mould is also growing. Mould is inhibited by the nitrates found in groundwater, the cause of rising damp, thereby eliminating rising damp as the root cause.
The sub-floor humidity was over 85%RH, a sign of water coming in. Note the humidity was too low for rot.
Looking immediately outside, we can see an ill-fitting junction between a square downpipe and round drain.
The patio is also cracked. One or both of these issues is causing rainwater to penetrate into the property.
The thermal image picks up heat loss, between the cold blue and warmer orange.
Dampness was detectable dropping down from the window-sill.
I tested walls in radio frequency mode. Water reflects radio waves at a set frequency similar to mobile phone shields. Meters can’t differentiate moisture from other dense matter such as metal and concrete. They help trace damp in a normal, homogeneous wall.
Readings below 300 REL indicate that a wall is dry below the surface. These meters are for rapid scanning, mapping and profiling, see https://surveyor.tips/profiling.
Immediately outside we see a crack in the windowsill that needs filling, see recommendations.
Note the blistered external render. This results from calcium sulphate, a key ingredient in cement, dissolving and crystallising under the render and or masonry paint.
The effect is noticeable on the rear wall. It is a result of a rain on bricks covered with an impermeable paint.
The salts are benign and can be removed with a wire brush.
The thermal image of the bedroom wall under the window highlights replacement of the door with a window.
Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. See https://surveyor.tips/mould. The secondary cause of high relative humidity is low temperature relative to the source of humidity, see https://survey.tips/humidity.
There are no trickle vents. Consider retrofitting trickle vents for increased background ventilation.
A simple alternative to trickle vents, is to open up the chimney flue.
Other matters
A plant is stick-up over the rear gutter.
Gutters are presumably a shared responsibility with your upstairs neighbour. Look to see the state of the gutters from the first-floor window. Observe and film around the property during a rainstorm.
Consider splitting costs with upstairs neighbour of replacing the ill-fitting square downpipe with a round one.
Monitoring relative humidity
I encourage homeowners with property showing signs of damp or mould to monitor the relative humidity. The data can be used to improve ventilation, increase heat balance, insulation and improve airflow through reduced clutter to see the benefits.
In this examples there is constant heat, but steady increase in humidity overnight. This suggests insufficient background ventilation. The simplest solution is to use a continuous flow extractor fan. Consider also using a PIV if you need filtered air, see surveyor.tips/piv.