How to join onto an existing building

With land in ever increasing demand, we are seeing more new builds planned on sites next to, or between existing buildings.
This can be an effective use of space; however, if the new build is attached to the existing property there are several considerations to take into account.

Potential issues

  1. Existing foundations - The existing wall and/or the foundation may be structurally unsound.
  2. Differential movement - 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. Soundproofing - The existing wall may not adequately resist the passage of sound.
  4. Damp - The existing wall may lack the necessary damp proofing.
  5. Legal - 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 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 (excluding Scotland) in place, drawn up in accordance with the Party Wall Act etc.  This requirement will only be relevant where the applicant is not the owner of the adjoining property.
  2. The separating wall must meet the relevant requirements of Building Regulations (for structural stability and sound resistance). Details and specifications to confirm how they achieve compliance will be required and a Structural Engineer’s report may be necessary 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 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.
  5. An effective ‘horizontal’ damp proof course (DPC) should be present in the existing wall which must be linked to the new DPCs and damp proof membrane (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.
  7. As an alternative to the above it may be feasible to design the proposed house so 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.

Examples of acceptable junction details


You can find more information on joining new builds to existing structures in Chapter 12.2 of our Technical Manual.

Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written 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. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.

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