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UK House Extension Episode 3 – Design and Party Wall Awards

Updated: Apr 26

Now that the preparation phase of your house extension is done, it will move into a more formal phase where you would actually be contracting out work to professionals. The first and a key contract you will decide is who to entrust the design and approval of your house extension to.


The design company will provide the following services to you:

  • Design Advice helping you shape your House extension

  • Full site survey where the designer will visit your house to take site measurements

  • Prepare planning permission drawings for approval and liaison with council for either neighbourhood consultation scheme, permitted development or full planning permission. This will include existing and proposed ground floor plan, site plan, roof plans, section details, side elevations, etc.

  • Rework the planning application if rejected to resolve the reasons for rejections

  • Building regulation drawings which will be required by building inspector and your builder showing all the steel beam placements and specifications

  • Structural calculations which will provide the specifications for the steel beams, rafters, floor joists which may be asked by the building inspector

  • Thames water build over agreement in case you are building over a sewer pipe or have to relocate a manhole. This is normally the case if you are extending a terraced house.

  • The design architect will create the specifications of the build which will also go as an input for getting quotations from the builders.

  • While you wait for the planning approvals, now is the time to get preliminary quotes from the builders. You might also need to start thinking about Party wall agreements.

Party Wall Agreement


This is where you now need to engage with your neighbours. If its a terraced house you will have to engage with both the neighbours for Party wall agreement. If its detached or semi-detached property, depending on what you are doing, you may need to engage with the neighbours.


What is Party Wall


A party wall is the shared wall, usually between a terraced or semi-detached house, and which divides the homes of two separate owners.


Before you start construction you need a Party wall agreement with your affected neighbours or you will need to appoint a Surveyor to prepare a Party wall award. Either one of this will be required before you start your build and most reputed builders will not commence work without this in place. The process is started by serving a Party wall notice on your neighbours in writing.

There are ample of websites out there giving advice on Party wall agreements and notice formats which we will not go into detail here but key points are:

  • Your neighbours can just sign the Party wall agreement allowing you to proceed with the build. This, by far, will be the cheapest and quickest option. But please be aware that this will leave you open to disputes later on if your neighbour thinks your construction work has caused damage to their property and that you should make good the damage. Cheap option but largely depends on relations with your neighbours.


  • Your neighbours will want a formal Party wall award. This is where you will now need to engage a surveyor. The Surveyor will survey your neighbours property before the work and after the work making notes and taking photos as appropriate. This ensures that any damage claims can be evaluated correctly to see if it was caused by your construction. Now this is where multiple options will present and how much it will cost you again depends on your neighbours. They do hold all the cards!

  1. You tell your neighbour your Surveyor company and the neighbour agrees to use the same as a joint surveyor. Remember you have to do this with both neighbours for a terraced house. This will cost you anywhere around £800 to £1200 per neighbour.

  2. Your neighbour will not agree to use your Surveyor and proposes their own Surveyor. However, the neighbour agrees to use their Surveyor as a Joint Surveyor. Cost wise this will be the same as above.

  3. Your neighbour will not agree to use your Surveyor and will also not agree to use theirs as Joint Surveyor. And further worst case is if both of your neighbours take the same stance (for terraced property). You then are looking at doubling of your expenses with worst case costing you £1600 to £2400 per neighbour.

So bottom line is it will absolutely pay to have neighbours on your side!


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