I'm considering buying a place that has a few problems, and I'm trying to figure out how difficult/expensive they'd be to remedy, and if it's worth it or not.

It's an old fishing lodge and is totally made of timber, other than one brick built boiler room set into the foundations that level out the slope it's on. It was built in the 70s.

A large proportion has had further thin timber cladding added at a later date so it's hard to assess the condition of the timbers behind that, but some of the original timbers can be seen and are rotting in a couple of places (I also saw some wasps come out from under one part). It does look like there may be some insulation behind it, rather than being 'single skin', but what I can see looks like it's encased in plastic, which I would have though would sweat against wood. I'm not sure if this is original or not

I don't know much about this kind of construction, but from the looks of it, it's essentially logs cut so that they stack up and interlock, and it's presumably quite hard to swap out one at the bottom, and potentially quite specialist work?

Ultimately, I'd like to fit a heat pump so presumably (unless timber is a much better insulator than I realise?) I'd need to remove all the existing cladding, replace any bad timbers in the main structure, then have a layer of modern, high performance insulation and cladding added. Presumably something composite is best for the cladding, and I'd want it to be highly fire retardant.

The roof would need similarly insulating - this looks to be some kind of bitumen like substance - it's the original roof and looks to be solid enough, but of course is 50 years old and won't last forever.

There's also an old oil tank in the boiler room - it's too big to fit through the door so would need cutting up to get it out, but oil reside will make it flammable, so is it practically possible to do this at all? It doesn't cause a problem per se, but wastes a lot of potential storage space

I've attached a few picture showing the construction, and the sort of rot I'm seeing, and also a rough floor plan with approximate dimensions.

Does anyone have a feel for the difficulty of doing this, and what sort of ballpark costs are likely to be involved? Should I just walk away?

edit - this picture doesn't seem to show up very well, but the whole place is about 19 metres wide by 6 metres deep, single storey

Here's a rotten outside part with what may be insulation showing

Welcome to the forum.

It sounds like an interesting project, but not one I would like to take on! Whereabouts is it, roughly. Is it in Britain? One thing you'll need to consider and become grindingly familiar with are the building regs and probaly planning permission. Planning permission is about appearances; building regs are about structure, and fire risk, and lots more.

If it has proper permissions, it might be cheaper to knock it down and build something to current standards or better. I don't know how much could be reused.

I suggest you check out what needs building regs approval. Plumbing, electrics, roof coverings, insulation come to mind.

I think it will be more expensive to do it up than to start again, but that's just my opinion.

It looks like the construction is on a concrete or block base using a straightforward stud wall clad with "log-lap " a type of decorative ship lap cladding, on both sides, I'm guessing.
The log-lap is readily available and replacing thew bottom boards is relatively easy. Doing it all round the house with new treated boards wouldn't be too difficult. The roof structure looks strong but maybe lacking insulation. Adding that externally,-warm roof, with a tin type covering perhaps an option? I have no idea how planning and building regs would view any re working of the structure though.

edit; X posted with Peter above

As others have mentioned, what is it currently classified to be used as? And what do you intend to use it for?

Old oil tank - we had one, left the hatch and drain open for a long time to be sure no vapour remained, then sawed it up.

Never realised before, that 'Material Change of Use' in Bldg Regs has suprisingly lax requirements - I'd assumed that MCU would be treated as newbuild (even more surprising, discovered that this applies to Part Q barn conversions).

It looks damn substantial (and not 'in the wilds' but right next to a 'proper' house?) for anyone to argue that it's a temporary building, or seasonal occupation, unless already clearly documented as such for Planning and/or BldgRegs.

I think all depends on the condition, overall, of the structural frame/framing (incl wall studwork?). If limited-area rot can be repaired, then go ahead, remove all cladding/insulation/membranes, maybe retain the internal lining, otherwise re-do it to high, fully breathable (no vapour barrier (VCL) reqd), highly airtight, ventilated-cladding/roofing standard. That will deal with interstitial condensation, penetration thro the cladding and penetration around windows as cause(s) of the persistent dampness that causes rot; I wouldn't trust any lesser standard (via lax MCU requirements) to do that trick reliably. If any other cause e.g rising damp (DPC), that has to be dealt with too.

This approach will at least move the external wall and roof surface outboard, which would need Planning consent, unless you're game for contorted arguments and attempts to fit within Permitted Development.

Armed with such best understanding, you may be in a good position to acquire at a good price, where other buyers (or even everyday surveyors) are put off by the unknowns - great to know that you'll just be sweeping away most of the dodgy build-up. Certainly play on the latter fears with the Estate Agent!

You'd be taken for a ride if that was 6 figure work

If youre seriously interested, pay a local builder to look at it with you to work out the build up and ball park costs