How to fill and insulate the cavity between two houses?

Hello everyone,

next week we will be producing the floor slab and then immediately start building up the walls.

We are now building an extension profile-aligned to an existing building. However, on the backside of the existing building there is an approximately 35 cm wide offset between the two houses (over a length of about 4.5 m). That means the existing building is set back 35 cm from the property boundary. This logically creates a gap between the two buildings, since we want to build along the property boundary. We planned the party wall with a sound insulating 24 cm brick, which does not provide sufficient thermal insulation over the length of the offset (the party wall is basically an outer wall over 4.5 m across the entire building height). The proposal from the shell builder was to work with a 36.5 cm outer brick over that length. But this would make my interior 12 cm narrower. And I really don’t want to give up any width.

The architect has now proposed to continue with the 24 cm brick and to build a supplementary wall (insulating brick 12 or 18 cm) on the neighbor’s property. The current neighboring owner would grant me an easement for this. And a potentially future owner could then demolish the supplementary wall at their own expense and rebuild it along my house at the property boundary.

I don’t think this idea is bad, but additional costs of about 7,000 euros will come up (after I already had to underpin the neighbor’s house for 10,000 euros).

My question is therefore whether you know of a building material that meets fire protection regulations and could be cheaply installed in the cavity?

Possibly letting the 35.6 cm brick encroach onto the neighbor’s property would also be an option. But that wouldn’t be much cheaper either, since I would basically have to buy the 12 cm of land for a proper regulation. Then there would be surveying costs etc. again, and there would always be an offset on the property boundary.

Or do you have other ideas? Attached is a picture of the offset. Overall, it is about 45 sqm of wall surface.

Best regards Martin

Sounds complicated.

If I see it correctly, I would also tend towards the 36.5 brick and go onto the neighboring property.

Of course, the question remains when everything was surveyed. How exactly was it surveyed. 12 cm probably still falls within tolerance limits. Today's surveys are much more accurate than, for example, 30 years ago. Although there will always be some inaccuracies.

If you agree internally with the neighbor, I would simply build like that. If later any people want to dispute the boundary line, they are welcome to pay for the surveying and court costs.

The outcome of a new survey is likely uncertain.

12 cm is practically nothing when it comes to plots of land.

Now we are building an extension to an existing building with the same profile. However, on the back of the existing building there is an approximately 35cm wide offset between the two houses (over a length of about 4.5m). That means the existing building is set back 35cm from the [U] . This logically creates a gap between the two buildings, since we want to build along the property boundary.

The gap was not yet present in the drawings (?), and if you are building with the same profile, wouldn’t it be better to cover the "mega joint" and fill the cavity with insulation?

Wouldn't it be more appropriate to cover the "Megafuge" and fill the cavity with insulation?

But that must be done very, very carefully and regularly checked. We rented a semi-detached house from the 90s where the covering was never inspected and the insulation up to the first floor became soaked with water. The moisture problem in the basement was misinterpreted, and when mold appeared on the stairs in the ground floor in both halves of the house, the extent became clear. No idea how it was ultimately renovated.

The gap was not yet present in the drawings (?), and if you build with the same profile, wouldn't it be better to cover the "mega joint" and fill the cavity with insulation?

That was still the submission plan. The offset there was already visible due to the two buildings of different heights.

Filling it with insulation was, I believe, the first thought. But it’s not entirely unproblematic due to fire resistance and the required amount. Basically, the area should be made so that it stays dry from above. The extra cost from the 24 cm to the 36.5 cm is also not insignificant. But something has to be done, and for 2000 euros it's probably not a big deal anymore.

I wouldn't have thought that 12 cm of land encroachment is irrelevant now. I would prefer a clear regulation for future legal successors.