This technical update provides additional guidance on new homes built alongside existing homes. It is important that all workmanship carried out during construction is completed in accordance with the relevant tolerances.
A question often asked is: Where a new home is built onto the side of an existing home, can we provide warranty cover and is it necessary for a new leaf of wall to be constructed on an independent foundation adjacent the separating wall?
The following information is intended to clarify what our requirements are for newly built "attached" dwellings.
1. The existing wall and/or the foundation may be structurally unsound.
2. If the two homes are not constructed independently there could be differential movement: this could cause structural damage to one or both homes and/or affect the weatherproofing.
3. The existing wall may not adequately resist the passage of sound.
4. The existing wall may lack the necessary damp proofing.
5. Our New Homes policy would not exclude the existing wall from warranty cover, as it would diminish the cover for the homeowner and even if it was possible, it may not negate all issues.
6. If a new leaf is not to be built, it is necessary to demonstrate that the owner of the existing wall recognises that the existing wall is to become a party wall.
In most cases the new home should be an independent structure (this would also be our preference) with a new wall constructed alongside the existing wall. The new wall should be supported on a new foundation, independent of the existing foundation, which may require engineer design.
However, there may be situations where it is possible to retain the existing external wall as a new separating wall. In these cases:
1. There should be a Party Wall Agreement in place, drawn up in accordance with the Party Wall Act and this should be made available to our underwriters. Please note that this requirement will be relevant where the applicant is not the owner of the adjoining property.
2. Confirmation will be required that the separating wall meets the relevant requirements of the Building Regulations, i.e. for structural stability and sound resistance. Details and specification to confirm how they achieve compliance will be required. A structural engineer’s report may be necessary in some cases to prove the structural stability of the wall.
3. The existing foundations and wall structure should be exposed to determine whether they are suitable to support any proposed increased loading resulting from the construction of the new home. A structural engineer’s report may be required to confirm the adequacy of the existing construction.
4. The junction of the new walls to the existing walls should ensure that dampness cannot track back into the new home or the existing home. (See suggested solutions below in Fig 1 and Fig 2.)
5. An effective ‘horizontal’ DPC should be present in the existing wall which must be linked to the new DPC and DPM of the new home.
6. At the junction of the existing and new structures, detailing should allow for differential movement without cracking. Any settlement should be limited to 2-3mm (which would not normally adversely affect the roof covering). A structural engineer’s report on how this can be achieved will be required, which must take account of the soil conditions, depth and type bonding proposed.
As an alternative to the above it may be feasible to design the proposed house such that no additional loads are applied to the existing wall. For example, the new house may be formed from steel or timber frame construction and has no reliance on the existing building for support or restraint.
For more information relating to the above please see Appendix D of the Technical Manual.
Figure 1: bonding new walls to existing solid masonry walls
Figure 2: bonding new walls to existing cavity masonry walls
Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.