Drellingore

That's a fair point. I'm not sure what the cut-off of likelihood is though. One might think that if the local water supply was taken offline for long enough for aquifer to fill then we'd have bigger problems than flooding! 😄

How can I prove that the risk of surface and groundwater flooding is lower than desktop surveys suggest? If I can't, what sort of mitigations might one expect to be required? Our desktop survey flagged high risk of surface water and groundwater flooding. The GOV.UK website says that groundwater flooding is "unlikely." The site has not flooded in the last 80 years, according to someone who was born there. The desktop surveys probably highlight a risk based on geology. There's an underground chalk river about 100m north of the site. For this reason the local water supply is extracted 300m away. As a result, it's almost certain that there'll never be any groundwater flooding, because it's being pumped out for drinking water on an industrial scale. In the distant past that underwater river would occasionally break ground level, roughly once a decade. It's not been seen here since the pumping station was installed. The architect is quite rightly pointing out that under their Principal Designer obligations that they need to be seen to be addressing the highlighted risk. I need to either: Prove that the risk is lower than the desktop survey suggests Figure out what mitigations might be required, and how expensive they'll be Any thoughts on the above? We'll leave out of the equation for now that apparently we're now in a very high radon risk area...

Quite probably!

'ello folks. Those familiar with my posts may have noticed me talk about our build being chosen for Grand Designs. I was very keen, the missus much less so. To spare you the details, she agreed begrudgingly because I'm insensitive and pushy, then wasn't happy, so I pulled the plug, but she's since had a (entirely voluntary!) change of mind and it's now back on. In a few different threads it popped up, and people had questions (like "why on Earth would you want to do that?!"), and voiced a few preconceptions that haven't been really matched with our experience thus far. Even our architect had a negative perception of the production process, which suffice to say the production team were surprised by. I don't think our contracts include any NDAs, but they do ask that we don't over-share the progress of the build, as that would spoil the narrative of the episode when it eventually airs. I figured I'd start a thread so if anyone's curious (and maybe is considering applying themselves) I can answer about the general process, without polluting other threads with off-topic stuff. About Grand Designs: All the people we've interacted with are very nice, and very considerate. They've been super patient with our internal debates, and they've been super-supportive of the missus and her concerns. They haven't (yet, it's early days) dictated anything to us about timelines. Their only asks have been 'if you can, tell us before something happens so we can decide whether to film it'. The perception that they'd insist on us delaying things for the benefit of their filming schedule seems unfounded. Sure, they said that they would ask if things could be moved if it'd help them out, but they also said that they wouldn't dream of telling people when they can/can't do works. They wouldn't have any leverage to do so anyway! We haven't done anything with Big Kev yet, but the producers are lovely and so far have been unintrusive. Follow you around, ask some questions, and maybe ask 'can you do that whilst I get it from this angle.' In summary, based on our limited experiences so far, I think some of the negative perceptions are unfounded. Let's see if I say the same in a couple of years! But one of my motivations for this post was to help spread the word that so far, they seem good people. About the decision: I'm a bit of a show-off, so I like this kind of thing. I do keynote talks at conferences, host a podcast for work, used to teach martial arts, so I'm comfortable being in front of people. It seemed like a great way of sharing the build and the site. I find it motivating knowing that other people will see what we're doing, and that somehow makes it more real to me. I found it validating that the show wanted to film the build. It felt a bit like 'winning', and that someone thought what we are doing is cool. It'll be good to document it for ourselves, the kids, and any grandchildren. I'd thought about trying to film it myself and doing a YouTube channel, but having done a podcast for work I can now see that'd be a boatload of work to do to an acceptable standard. The missus works in the industry, and is studying for her Part 2 in architecture. The exposure will, I'm certain, be great for her career. Even if she doesn't yet agree! You get a letter to prove that they're filming you, so you can tell suppliers that they might be featured. Each build gets a page on their site, and you can list key suppliers. This should help us secure discounts, and also motivate folks to not arse-up the build - I don't think many people want to be seen as a massive problem on national telly. We've already had some doors open as a result of their involvement. So basically, what we have learned from this is that I'm insensitive and needy of external validation 😆

Full mitigation means sumps and the like, as far as I understand? Again. Mx. GPT reckons this is a few grand, but the architects and Visqueen imply that it'll be non-trivial.

Ergh. BGS survey came back with >30% probability, so that'll be full mitigation.

Kent, north of Folkestone. My username is the location... Although I should have pretended to be Dr. Ellingore! It's very old, very not-flat, and we need to insulate under it otherwise we lose a lot of space. That's my understanding anyway; I tend to take the wife's word and the architect's word on such matters. We do. They're saying radon is outside of their remit.

How much more expensive is it to put in radon-graded DPM and ensure it's sealed properly? We're converting an existing barn, and replacing the concrete slab. Presumably we'll need to be careful about details where the DPM meets wall foundations underground, but then I'd've expected that anyway to keep the damp out? My architect is suggesting that it's going to be an expensive faff. My naïve web searching and conversations with Mx. GPT suggest otherwise. BuildHub is normally a good source of strong opinions - any thoughts? 😄

I think this might be a bit of a stretch - someone looks at either the radonUK map or BGS data, and tells you what the number is. I've gone down the route of a BGS GeoReport as recommended by @kandgmitchell because our site doesn't have an address yet, so isn't eligible for the radonUK reports. The BGS ones work on coordinates, and are 25m resolution, whereas I believe the public data is 1km resolution. I'd agree that planning enforcing building control concerns is a bit daft. But then, that's not very surprising.

Thanks for the very detailed and considered thoughts, @Gus Potter. This insurance malarkey sounds a lot more complicated than I was expecting... And I don't know why I was expecting it to be simple! We need the public liability insurance for the Grand Designs film crew. I think getting that in place as a separate item, and then giving the overall package more thought is the way to go. We do have an architect and an SE involved, so hopefully that makes things easier and the exposure smaller.

Anyone got any recommendations for folks that can provide only public liability insurance for a barn conversion site? Protek are asking for a list of stuff that's longer than both my arms and my legs - like estimated costs to rebuild the 18th century barn back to its prior state. They're refusing to separate out the public liability insurance, which I need by Wednesday 20th ideally. Instead they want to quote for all aspects of insurance. When I've done this for my IT consultancy businesses, some of which had their own offices, this was a trivial box-ticking exercise and I'm pretty sure was just a matter of filling an online form. Are any of y'all aware of someone that makes it this simple?

I went down the route of getting a licence for about €1k, and it's a reasonably complicated app for which you need a lot of data - if you've not got the data to put into it, you won't get any value from it. I used ChatGPT and Gemini Deep Research to infer values for moisture curves and the like. Our architect and I designed a build-up to model, and it passed - yay! But then I changed the colour of the external wall and it failed. Then switching the orientation from north to south went from one failure to 48. There are a lot of things that I think I can do better myself, and this isn't one of them. That, or the build-up we're proposing really is terrible.

Because I've already done it for him.

So I've told him that I'm not paying the remaining 25% of the fee because the deliverables contained significant undisclosed errors. Am I being unreasonable?

Does Penny ask you to punch a lot of people? 😆

If you care about the pennies, the pounds look after themselves! Or something.

We think it'll be fine - the hemp block is apparently more water-permeable than wood fibre for the same U-value. It's a shame I can't do it on the free app, because getting additional confidence would be worth paying the price of an hour of ChatGPT guiding me through the process. I'm not sure that peace of mind will be worth a €1k software licence though!

Ta @MikeSharp01! Can you make a recommendation of the folks you used? I'm going to ask the architect to ask the supplier if they want to contribute towards the cost, seeing as the information will be useful for their future sales and marketing.

I don't suppose anyone here has a copy of WUFI Pro and would be willing to run some calcs for me, would they? The architect wants to use hemp block as part of the build-up for some flint-and-lime walls. The manufacturer doesn't have WUFI data available, so we were going to run the calcs ourselves to make sure we won't ha ve a problem with condensation. Only problem is that the free version of their software doesn't allow you to specify custom materials, which means I'd need to pay €1k for a year-long license when it's only a quick job that needs doing.

I'd be inclined to agree with this, but in this instance you're not allowed to see the PHPP model itself unless you also buy a licence. So this calculation is normally 'behind the curtain'. Removing them drops the heat demand by 6kWh(m²a), or about 28%.

Case in point: when doing SAP calculations, I just noticed that the Passivhaus consultant appears to have accidentally added over 100sqm of extra walls that don't exist to the property...

We need a 'tin hat' on a 27Lx10Wx8H metre building. Good point!

Is it normal for trades like scaffolders to offer cash discounts? Coming from the world of enterprise, it seems a bit dodgy. I've never withdrawn £20k in cash before...

There's a few reasons: You get a letter so suppliers can say "as featured on Grand Designs," which allows you to negotiate discounts Suppliers don't want to look bad on national telly, so give you better treatment If we Airbnb the second building, then we've just got a free advert on national TV It'll be nice for the kids to be able to watch and one day hopefully show their kids The barn is a non-designated heritage asset, and so it'll be nice for future generations to know about this stage of its evolution It's much less work for us than trying to self-document the journey as a vlog or something The wife works in interior design and interior architecture, so it'll be great promotion for her career We used to watch Grand Designs when we first got together nearly twenty years ago, thinking "I'll never be able to afford to do that." So it's kinda nice being able to say that now we've 'done it' (obviously assuming the project succeeds!)

Nah. What I've learned this far in the project is that I do a better job than most (but not all) of the professionals we've paid. If I have to do the legwork to then hand it over to someone qualified to rubber-stamp, that's better than spending weeks waiting for someone who has multiple priorities and little incentive to come up with novel solutions. It seems the construction industry is full of people that only do things the way they've always done them, and think everything else is impossible.