Serious Mould problem - advice needed

[Biosblbay]

Moved into our home in October 2024 so a little under a year now. My Mum is living in a self contained annexe at the back of our garden, but when we bought this property it was previously used as a studio for a business and was more like a garage type space with a toilet. This has since been upgraded as a 1 bedroom annexe with fully working toilet and kitchen and has been signed off by the council as a proper self contained annexe. This cost a lot of money and was only completed in April this year so it’s all very new. My mum has just made us aware of the mould she has growing in her annexe. It is horrendous, spreading all over her furniture, up walls, growing on places you just wouldn’t believe! We knew she had a small amount of it up one wall which we didn’t think would be too much of an issue but she’s just moved some things about and it’s everywhere!

She has air vents, windows, doors etc so there is ventilation. Any ideas why this mould would be growing? Where could it be growing from for it to be spreading all over? And the most important what do we even do to tackle such a bad mould issue?!

any advice/ help would be very much appreciated as I have no idea what to do. I am just hoping the whole thing doesn’t need to be knocked down, it cost us £24k just to change some internal walls and fit in a bathroom, kitchen and some extra windows. We can’t afford to do much more!

It must be damp - we have outbuildings and they’re damp
we had to have the whole floor ripped out as there was no appropriate damp proofing under the flooring or drainage so it was in a poor state
has it been built as a space to be lived in vs an office ?

@rubyslippers it was used as a business before and was not lived in, but we just assumed that the council would have checked all this before they signed it off as a self dwelling annexe or the builders would have maybe said something about it. The only thing we didn’t change was the flooring in the living area, could that maybe be the issue? It is a lino floor but no idea what is under it. Is it maybe worth us changing the living room floor?

If there’s nothing between the floor and ground it will be an isue
ig needs waterproofing etc
Foundations are key
your builders should have known this as it’s diff from a domestic dwelling

You must move your mum into your house! It's really bad for you to be living in such mouldy conditions.

Was it an out building like a coach house or garage? How old was the building? Is it old enough to have had a damp roof course put in? Did the builders put a damp proof in under the main floor?

@Geneticsbunny it was a garage many years ago but was knocked down and rebuilt in 1996, has had 2 previous owners, but would have no idea about the flooring

@canyon2000 she’s just sent me some more photos and it it really really bad! I am in shock. I will post the photos. I have no idea why this would be happening, it seemed to be ok when she first moved in, but it’s spreading like wildfire!

Hi is there any chance you could post the photos so I could try and determine what's causing it?
Does it have single skin walls by any chance?

This is the mould in the annexe. It seems to be coming from the floor! The photo frame and cat scratch post also have mould and these were on the floor. @rubyslippers@Geneticsbunny

@Ladybuilder just posted some photos. They are currently being reviewed before they are posted. It’s pretty bad! It seems it might be from the floor as others have mentioned

What’s the construction of the building? Is it brick? If so is it built in proper foundation and does it have damp proof course?

@Ladybuilder here is another 2 photos. It’s gone all up her white velvet chair, on this basket. Seems to be anything touching the floor but I could be wrong.

Do you have photos of the walls / floor and inside outside.

If it's very damp then it could be lots of things and it might link to a construction issue.

Does it have cavity walls?
Flat or pitched roof?
Solid or suspended floor?
What's around the outside, have you paved up to it for example?

The council sign off for building regs and there will be a checklist and they should've i spected at various stages of the build but that doesn't prevent builders doing things wrong or using incorrect materials.

My bet is that the walls aren’t insulated enough coupled with not enough insulation from floor to wall. I think you need to get a second opinion from a different builder. And in the meantime open it up completely, lots of air throughout and put heaters on full blast with dehumidifiers.

@Pepperedpickles we’ve just booked for a mould specialist to come in. Quoted us £1195 😩 but that might not even solve the issue completely! But I reckon it’s the floors too and maybe the walls combined. What a nightmare!

I agree with @Pepperedpickles. There’s a lot of moisture in there for mould to grow like that!

That is totally uninhabitable
I suspect you will need to pull it apart to work out where the issue is coming from. Hopefully that mould specialist will enable to help but I supect that damp is either trapped in the walls or floor or both and that a lack of insulation is making the problems worse.

If it was a garage then I wouldn't have thought they would have put a membrane In underneath the concrete slab and they definitely won't have insulated it so that is an obvious area to check.

Then I would look to see if you can see a damp proof course. It should be in between the bricks horizontally above ground level. Again if it was built as a garage it probably doesn't have this.

And I am guessing that it is single skin because it is a garage. How much insulation did they put in the internal walls when they were fitting it out?

So I suspect you have damp coming up through the floor, and poorly insulated walls and ceiling which means that moisture is condensing on these cold surfaces.