The national extension rules assess the relevant dwellinghouse, while the Party Wall Act addresses specified work and adjoining owners' rights. [1] [2]
Agree the intended arrangement first
Prepare a combined drawing showing both existing houses and both proposed additions. Then identify what each owner is obtaining, what construction is shared and what remains independent. Agree which drawings each person is approving and how later changes will be authorised.
For example, decide whether a new dividing wall is to be wholly on one property or astride the boundary. These arrangements have different consent implications. A new wall astride the boundary requires written consent; the government's booklet recommends recording its location, costs and agreed terms. [2]
Check planning for the complete proposal
Both houses need a lawful planning route. For each one, identify the original footprint, earlier extensions, relevant boundaries and any removed permitted development rights. Do not add the two houses' dimensional allowances together or assume that a neighbour's consent makes an otherwise excessive extension permitted development. [1]
For a planning application, show the whole coordinated scheme and discuss the appropriate application arrangement with the council. Use that council's current design guidance. A larger joint design is not a generally available national entitlement merely because it looks balanced across two plots. [3] [1]
Ask the council to clarify how it would assess the proposal if one owner did not build. Do not rely on a planning outcome that assumes simultaneous construction without understanding how that assumption is addressed.
Plan for one project changing
Before committing, record how the project will work if one owner changes their design, funding or start date. Ask the design team to identify any part that cannot safely or practically be built independently.
The structural engineer should understand the full arrangement, including any shared support or foundation works. Relevant existing construction may need investigation, and the load path needs to be established rather than inferred from two matching floor plans. [4]
Put responsibilities and money in writing
Set out who appoints the designers and contractor, who approves variations and which owner pays each part of the work. Use a written, itemised scope and an agreed method of dealing with changes. RICS recommends documenting the work, price, dates and variation procedure for home-extension contracts. [5]
Check insurance arrangements for work affecting both properties. Tell the relevant insurers what is proposed rather than assuming one builder's policy covers every shared risk. [5]
Keep statutory procedures separate
Party wall notices and any required award still need to cover the actual works. A shared desire to extend does not remove obligations to protect adjoining property or deal with damage. [2]
Nearby sewers also need their own assessment: a joint design does not authorise building over public infrastructure. Confirm the drainage proposal before making a common wall or roof line dependent on it. [6]
A solicitor can help document shared rights and obligations where the arrangement goes beyond a straightforward, independent extension to each house.