A Complete UK Identification Guide
Black, green, white, or that musty smell with nothing visible yet — here’s how to identify the mould in your home, which types are genuinely dangerous, and when DIY cleaning isn’t enough.
There are more than 100,000 known species of mould, but only a small handful regularly turn up in UK houses and flats. Most people only need to recognise a few visual patterns — colour, texture, and where it’s growing — to know roughly what they’re dealing with and how urgently it needs treating.
This guide covers every type of household mould you’re likely to encounter in a British property, organised by colour for quick identification and by species for anyone who wants the detail behind it — including which types are genuinely classed as toxic, and which are simply unsightly. If you’d rather skip straight to treatment, our professional mould removal service covers diagnosis through to a guaranteed result.
Why does mould grow in UK homes?
Mould is a fungus, and every species needs the same three things to establish a colony: a moisture source, an organic food source (dust, wood, plaster, fabric, or wallpaper paste all qualify), and a surface temperature usually between 15–30°C. UK housing stock — particularly solid-wall Victorian and Edwardian terraces — provides all three more readily than most people realise.
The moisture itself almost always comes from one of three sources, and which one matters because it changes the fix:
- Condensation — warm, moist indoor air meeting a cold surface (a north-facing wall, a single-glazed window, behind a wardrobe). This is the most common cause of mould in London flats and houses, particularly in winter.
- Penetrating damp — water entering through a defect in the building’s fabric, such as a cracked render, faulty guttering, or a leaking roof. See our guide on penetrating damp for the signs.
- Rising damp — groundwater drawn up through masonry via capillary action, usually affecting the lower metre of a wall in older properties without an effective damp-proof course. Compare the two in our guide to rising damp vs penetrating damp.
Mould itself doesn’t cause structural damage in most domestic cases — but it’s a reliable signal that one of the above is present and untreated, and persistent moisture absolutely does cause structural damage over time, from rotting timber to blown plaster.
Identifying mould by colour
Colour is the fastest way to narrow down what you’re looking at, though it’s not a definitive identifier on its own — the same species can vary in colour depending on its food source, age, and moisture level. Use this as a starting point, not a diagnosis.
Black mould
Usually Cladosporium (velvety, allergenic) rather than true toxic black mould. Found on walls, window frames, sealant, and fabrics in poorly ventilated rooms.
Green mould
Typically Cladosporium, Aspergillus, or Trichoderma. Fuzzy texture, found on damp wood, carpets, and food, in humid or poorly aired rooms.
White mould
Frequently a young colony of Aspergillus, Penicillium, or Cladosporium before it darkens. Fluffy texture; feeds on wood, paper, and plaster.
Blue / blue-green mould
Almost always Penicillium. Fuzzy, velvet-like. Found on water-damaged materials, mattresses, insulation, and rotting food.
Yellow / brown mould
Can be Aureobasidium, Aspergillus, or Serpula lacrymans (dry rot) in timber. Often linked to leaking plumbing — worth checking nearby pipework.
Pink “mould”
This is bacteria — Serratia marcescens — not fungus. Common in bathrooms around soap residue and grout. Harmless to most healthy adults but should still be cleaned promptly.
Mould species found in UK houses, explained
For anyone who wants more than a colour match, here’s what’s actually growing — based on the species the Centers for Disease Control and Prevention and UK building surveyors most commonly identify in domestic properties.
Cladosporium (the UK’s most common household mould)
Olive-green, brown, or black with a velvety, suede-like texture. The mould most people mean when they say “black mould” without realising it isn’t the toxic variety.
Penicillium (the blue/green mould)
Blue, green, or yellow with a fuzzy texture. Thrives on anything water-damaged — the same genus used in penicillin and blue cheese production, but a health hazard when growing uncontrolled indoors.
Alternaria (bathroom and sink mould)
Velvety texture, dark green, brown, or black. One of the fastest-spreading household moulds.
Aspergillus (attics, basements & drywall)
Green, white, or grey with dark spots and a powdery appearance. Extremely common, and some species produce mycotoxins.
Stachybotrys chartarum (true toxic black mould)
Slimy and dark greenish-black or greyish-black, with a strong musty smell — visually distinct from the dry, velvety look of Cladosporium. This is the species genuinely classed as toxic.
Aureobasidium (behind wallpaper and on paintwork)
Starts cream-coloured and darkens to brown or black over time.
Chaetomium (after major water damage)
Cottony white at first, darkening to grey or black. Often mistaken for toxic black mould by sight alone.
Trichoderma (damp carpets and timber)
White and green appearance; a common soil fungus that establishes indoors wherever organic, damp material is present.
| Species | Min. RH for growth* | Risk classification |
|---|---|---|
| Cladosporium spp. | ~70% RH | Moderate — allergenic |
| Aspergillus spp. | ~75% RH | Moderate — allergenic/pathogenic |
| Penicillium spp. | ~78% RH | Moderate — allergenic |
| Chaetomium spp. | ~80% RH | Moderate-high — allergenic/toxigenic |
| Stachybotrys chartarum | ~90% RH | High — HHSRS Category 1 |
*Minimum relative humidity at the wall surface required for spore germination at typical indoor temperatures (15–20°C). Source: EN ISO 13788 and BRE Report BR 262. This is the same assessment standard we use when conducting condensation risk surveys across London.
Allergenic, pathogenic & toxigenic mould — what’s the actual difference?
Beyond colour and species, every household mould falls into one (or more) of three functional categories based on how it affects the body. This is the classification system used by the Centers for Disease Control and Prevention, and it’s more useful for assessing risk than colour ever will be.
Allergenic
Triggers allergic reactions — sneezing, coughing, itchy eyes, worsened asthma — in sensitive individuals. The largest category; includes Cladosporium, Penicillium, and Alternaria. Generally not dangerous to healthy adults in small quantities.
Pathogenic
Capable of causing infection, particularly in people with weakened immune systems, chronic respiratory conditions, or open wounds. Some Aspergillus species fall here.
Toxigenic
Produces mycotoxins as a metabolic byproduct, which can cause more serious health effects with prolonged exposure. Stachybotrys chartarum is the best-known example, though some Aspergillus and Chaetomium species also qualify.
Is mould dangerous to your health?
Damp and mould are classified as a Category 1 hazard under the Housing Health and Safety Rating System (HHSRS) — the same classification used for asbestos and carbon monoxide. That doesn’t mean every patch of mould is an emergency, but it does mean it’s taken seriously in UK housing law, particularly for rented properties.
Common health effects linked to mould exposure include persistent coughing, sneezing, headaches, fatigue, skin irritation, and worsening of existing asthma or allergic conditions — typically improving when the person spends time away from the affected property. Toxigenic species can cause more serious reactions with prolonged exposure, particularly in children, the elderly, and those with compromised immune systems.
Where mould commonly grows in UK homes
The species often varies by room, because each room has a different combination of moisture, ventilation, and food source.
🛏️ Bedrooms
Cold external walls, behind wardrobes, and around window reveals — usually condensation-driven. Read our dedicated guide to bedroom mould causes and removal.
🛁 Bathrooms
Sealant, grout, and ceiling corners after showering without extraction. Often Alternaria or Cladosporium, sometimes pink bacterial growth around soap residue.
🍳 Kitchens
Around extractor fans, under sinks, and behind appliances — frequently linked to cooking steam and undetected pipe leaks.
🏠 Lofts & basements
Aspergillus and Chaetomium are common here, often following roof leaks or rising damp in below-ground spaces with poor airflow.
Do I need a professional, or can I treat it myself?
For a small, isolated patch — a corner of bathroom sealant, for instance — a shop-bought spray and improved ventilation is often sufficient. For mould on walls, ceilings, or inside plaster, particularly anything covering more than roughly an A4 sheet of paper, professional treatment is the only approach that reliably lasts, because it addresses the mycelium root structure beneath the surface, not just the visible colony.
| Professional treatment | DIY spray | |
|---|---|---|
| Penetration | Reaches 3–5mm into plaster, killing the root network | Surface-only; mycelium survives inside the wall |
| Moisture diagnosis | Root cause identified and addressed | No diagnosis; regrowth likely within weeks |
| Documentation | Written report — usable for landlord/insurance purposes | No record |
| Best suited to | Walls, ceilings, recurring mould, rented properties | Small, isolated, surface-level patches |
For the full breakdown, see our guide comparing DIY vs professional damp and mould treatment.
Related guides
The takeaway: fix the moisture, not just the mould
Whichever type of mould you’re looking at, the principle behind a lasting fix is identical: identify and resolve the moisture source — condensation, rising damp, or penetrating damp — then treat the existing growth properly. Cleaning the surface without addressing the cause buys you a few weeks at most. If mould keeps returning, covers a wide area, or you’re dealing with a landlord dispute that needs documentation, a professional survey is the quickest way to get a definitive answer.
Not sure what you’re looking at?
Send us a photo and we’ll give you an honest first opinion — no obligation, same-week surveys available across London.
Frequently asked questions
What is the most common type of mould found in UK houses?
Cladosporium is the most frequently found mould in UK homes, followed closely by Penicillium and Aspergillus. All three are allergenic moulds and typically appear black, green, blue, or grey on walls, fabrics, and window frames in poorly ventilated rooms. Genuine toxic black mould — Stachybotrys chartarum — is far less common and usually only develops after prolonged, severe water damage.
Do I need to know which species of mould I have before removing it?
No. Identifying the exact species is rarely necessary before cleanup, because the treatment principle is the same for almost every type: remove the growth correctly and fix the moisture source. Species identification matters most when mould is being used as evidence in a landlord dispute, insurance claim, or HHSRS complaint.
Is all black mould toxic?
No. Most mould that looks black is actually Cladosporium, an allergenic mould rather than a toxic one. True toxic black mould, Stachybotrys chartarum, has a distinctive slimy, dark greenish-black appearance and a strong musty smell, and only thrives in conditions of prolonged, severe dampness — typically above 90% relative humidity. A professional survey is the most reliable way to tell the two apart.
Can mould in a rented property be a landlord’s responsibility?
Yes. Under the Homes (Fitness for Human Habitation) Act 2018 and the HHSRS, damp and mould are classed as a Category 1 hazard, and landlords have a legal duty to address it. Tenants who report mould caused by structural issues or inadequate ventilation, rather than their own behaviour, can request a council inspection or pursue compensation if the landlord fails to act.
What’s the difference between mould and mildew?
Mildew is a surface-level fungus that typically appears as a flat, white or grey powdery patch on damp surfaces such as shower tiles or windowsills. Mould grows in greater depth, often has a fuzzy or slimy texture, and penetrates into the substrate it’s growing on, making it harder to remove permanently.
Why does mould keep coming back after I clean it?
Mould returns after cleaning when the underlying moisture source — condensation, rising damp, penetrating damp, or poor ventilation — hasn’t been addressed. Surface cleaning only removes the visible colony; the root network beneath the surface and the conditions that caused it remain unless professionally treated.