If your loft conversion affects a shared wall or boundary, the law requires a party wall agreement to keep everything above board. It’s there to protect you and your neighbour, setting out how the work will be carried out and what safeguards are in place.
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A party wall agreement is a legal document between neighbours who share a wall or boundary. It's required under the Party Wall Act 1996 when building work could affect the structure you both share.
This includes common loft conversion jobs like raising rooflines, installing steel beams, or cutting into a party wall. It also applies to extensions and basement projects.The agreement sets out the planned work, how it will be carried out, and how any risk or disruption will be managed. If your neighbour doesn’t consent, surveyors will step in to create a formal document called a party wall award.
In this guide, we’ll explain when you need one, what it costs, and how to handle the process with confidence.
Whether a loft conversion requires a party wall agreement depends on the type of work being carried out. Some projects directly affect shared walls or boundaries, while others, like simple rooflight installations, avoid major structural changes and fall outside the Party Wall Act.
A party wall agreement is normally needed when the conversion involves structural work to a shared wall or boundary. Here are some common examples:
In these situations, you’ll need to serve notice to your neighbour at least two months before work begins and either secure their consent or go through the surveyor process.
Not every loft conversion falls under the Party Wall Act. Velux loft conversion projects, for example, usually avoid structural changes to shared walls. If you’re simply adding roof windows, insulating the loft, or carrying out work that doesn’t touch a boundary wall, you generally won’t need an agreement.
It’s still important to check early in the planning stage. A professional architect or loft specialist can confirm whether your design triggers the Act, and if in doubt, applying for a Lawful Development Certificate offers peace of mind.
To explore the options available, see our guide to types of loft conversions.
If your loft conversion affects a shared wall, ceiling, or boundary, you’ll need to follow the procedure set out in the Party Wall etc. Act 1996. The process is designed to protect both you and your neighbour by making sure the work is carried out fairly and safely. Here’s how it works in practice.
The process starts with a formal party wall notice. This must be served in writing to every neighbour whose property shares a wall or boundary affected by the work. The notice should clearly describe:
You must give at least two months’ notice before construction begins. Even if you have a good relationship with your neighbours, a written notice is a legal requirement and provides proof that you’ve followed the correct process.
Once the notice is served, your neighbour has 14 days to reply in writing. If they give consent, you can move forward with your loft conversion once building regulations are in place. If they refuse or don’t respond, this automatically counts as a dispute.
It’s worth remembering that verbal agreements don’t hold any legal weight under the Act. Always ensure written confirmation is obtained, even if your neighbour is supportive.
If a dispute arises, surveyors must get involved. You can both agree on one surveyor (an “agreed surveyor”) to act impartially, or each appoint your own. The surveyor’s role is to:
Choosing an agreed surveyor is often faster and cheaper, but having separate surveyors is an option if trust is an issue.
Once surveyors have completed their review, they will issue a legal document known as a party wall award. This sets out:
A party wall award often includes a schedule of condition, a detailed record of your neighbour’s property before work begins. This is crucial if damage is alleged later. Once the award is in place, the loft conversion can proceed, but you must stick exactly to what has been agreed.
The cost of a party wall agreement can vary depending on the scope of your loft conversion and how your neighbours respond. In most cases, you’ll need to cover the fees for a surveyor, and sometimes more than one if there’s a dispute.
Here’s a breakdown of the costs involved.
The cost of a party wall agreement almost always falls to the homeowner carrying out the loft conversion. Under the Party Wall Act, you are responsible for covering the surveyor’s fees, even if your neighbour appoints their own. If both sides use a single “agreed surveyor,” the cost is lower, but if a dispute arises and each neighbour brings in a separate surveyor, the bill can increase significantly.
On average, expect to pay £700-£1,000 if consent is given quickly, and £1,500 or more per neighbour if multiple surveyors are needed. In rare cases, a third surveyor may also be appointed to resolve disagreements, which adds further cost. While this may seem one-sided, the reasoning is that you are the one benefiting from the work, so it’s your responsibility to ensure your neighbour isn’t left out of pocket.
Yes. If your neighbour doesn’t give consent to the works, a dispute is triggered. This means both parties will need their surveyor, which can double the cost. In some cases, a third surveyor may be appointed to assist in resolving the matter.
To avoid this, talk to your neighbours early. Show them your plans, offer to answer questions, and consider appointing a surveyor you both trust. Clear communication upfront can save time, money, and stress in the long run.
When a loft conversion involves work to a shared wall, the Party Wall etc. Act 1996 comes into play. This law is designed to protect both you and your neighbour when building work affects a wall, boundary, or structure you both rely on. For loft conversions, this usually involves party walls in terraced or semi-detached homes, as well as structural changes for dormers or mansard extensions in some cases.
In a terraced house, the party wall is the dividing wall between you and your neighbour. If your loft conversion requires cutting into this wall to insert steel beams, raise the gable end, or adjust the chimney stack, you’ll need a party wall agreement before work can start.
The process begins with serving a formal party wall notice at least two months before work starts. Your neighbour can either consent in writing or dissent, which means surveyors are appointed to draw up an agreement.
The agreement sets out how the work will be carried out, access rights, and how any damage will be made good. While this may feel like a formality, it’s an important safeguard for both sides and keeps the project legally compliant.
Dormer and mansard loft conversions often involve more structural changes than a simple Velux design. Building a dormer loft conversion on the rear roof slope, for example, may require cutting into the party wall to support new beams.
A mansard, which reshapes the roof, almost always affects the shared wall in a terraced or semi-detached setting.
In these cases, a party wall agreement isn’t just a box-ticking exercise. It ensures the new load is properly supported, your neighbour’s property is protected, and any future disputes are avoided.
Many loft specialists will advise getting an agreement in place early, as it’s a predictable requirement for these types of extensions.
Party wall agreements are one of the most common causes of delay in loft conversions, but they don’t have to be. The key is clear communication with your neighbours from the outset. Share your plans early, explain what’s involved, and reassure them about protections under the Party Wall Act.
If neighbours feel included, they’re more likely to give written consent, which avoids the need for surveyors and speeds up the process. If surveyors are needed, appointing an agreed surveyor (one acting for both parties) is usually quicker and cheaper than each side appointing their own.
By handling communication well, most loft conversions involving a party wall can proceed smoothly without conflict or unnecessary cost.
A party wall award is a legal document prepared when neighbours don’t agree to proposed building works. It’s drawn up by a party wall surveyor and sets out exactly what work can go ahead, how it should be carried out, and what steps must be taken to protect both properties.
It includes details such as working hours, access arrangements, and procedures for handling any damage. Once agreed, the award is legally binding and ensures the project can move forward fairly for everyone involved.
Thinking about a loft conversion but unsure how party wall agreements work? Here are the answers to the most common questions homeowners ask. We’ve kept it simple, accurate and straight to the point.
Not always. If your loft conversion affects a shared wall or boundary, like installing steel beams, building a dormer, or raising the roof, then yes, you’ll usually need one. Velux-only conversions that don’t alter the structure generally don’t require it.
You must give your neighbour at least two months’ notice before starting work. If they consent quickly, the process is straightforward. If a surveyor is needed, allow a few extra weeks to agree and issue the party wall award.
No. If your work falls under the Party Wall Act and you build without consent or an award, your neighbour could take legal action. This can lead to delays, fines, or even necessitate the reversal of the work. Always follow the proper process first.
A party wall notice is the formal written notice you serve to your neighbour before starting any work. If they don’t consent, a surveyor will be appointed to issue a party wall award, a legal document that outlines how and when the work can take place.
If you're carrying out the work, you usually pay for all surveyor fees. That includes your neighbour’s surveyor if they choose to appoint their own. It’s considered part of the cost of the project.
Yes, if your loft conversion affects a shared wall, boundary, or involves deep foundations, the law requires you to serve notice and, if needed, secure a party wall agreement or award before starting work.
If you plan to excavate within three metres of your neighbour’s foundations and your work will go deeper than theirs, you must serve a party wall notice. This often applies to projects involving new footings or heavy structural work.
A shared wall becomes a party wall when it separates two properties and is owned by both parties. Internal walls within your own home are not party walls. The key factor is joint ownership across a legal boundary.
