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Basement Extension Guide 

Basement Conversion


Basement conversions are an excellent way of adding all-important space to your property. Depending on the size and footprint of your home, they can allow for the addition of a living area, bedroom, gym or maybe even a swimming pool!

That being said, they are one of the most complex types of construction work you can undertake to your property. With this complexity comes risk. Part of mitigating that risk is going through the necessary Party Wall Surveying procedures.

We thought we would take a look at Basement Extensions through a Party Wall Surveying perspective in the hope of helping you understand the type of considerations that Party Wall Surveyors will look at if they act for you or your neighbour.

Security Expenses

Security for Expenses

Security for Expenses is formally dealt with under Section 12 of the Party Wall etc Act 1996 and confirms that an adjoining owner (the neighbouring owner) may serve a formal Notice upon the building owner (the owner undertaking the works) requesting a sum of money, known as the “security” to be held on account during the course of works.

The security is in place to safeguard the adjoining owner in a situation whereby the works start and then remain unfinished for a prolonged period of time, or if the damage is caused to the adjoining owner’s property. The funds can then be called upon to enable to the adjoining owner to safeguard or repair their structure.

In our experience Security for Expenses funds are usually held on account during the works and for a specified amount of time (usually 1 to 6 months) post works.

With Security for Expenses figures often starting from £10,000, this isn’t a figure to overlook.

Special Foundations

Special Foundations

Through the eyes of the Party Wall etc Act 1996, there are two types of foundation that can be used to construct the new subterranean basement walls.

The first type is mass concrete, which is a thick column of concrete beneath the full thickness of the existing party wall and corbel footing.

The second type is a narrower column of concrete reinforced with metal. This is known as a special foundation.

In order to construct a special foundation, the building owner will require the consent of the adjoining owner. Obtaining this consent can often be a precarious task and we would advise discussing this at the earliest opportunity to avoid late-stage design changes and delay to the works on site.

Movement Monitoring

Movement Monitoring

Movement Monitoring systems ensure that any movement to the adjoining owner’s structure is detected at the earliest stage possible, while also ensuring there is a full record of the movement in place in the event of issue or damage.

There are a number of different approaches to movement monitoring, however, generally speaking, monitoring targets will be installed to the adjoining owner’s property before the building owner commences their construction work. Generally, these will be installed to a number of different predetermined points on the property such as the front, rear or flank walls.

Weekly movement reports will then be produced and provided to the Party Wall Surveyors to ensure that movement “trigger points” haven’t been reached.

Movement Monitoring systems will often be calibrated to detect as little as 1mm movement. Being able to detect early movement means the movement issue can be addressed in the first instance and in advance of becoming or leading to a severe issue.

You can see the Monitoring targets in red on the drawing above.

Surveying advisers or engineers

Advising Engineers

Advising engineers are usually called upon by the adjoining owner’s Party Wall Surveyor.

The advising engineer’s role will be to review the building owner’s structural design, calculations, and specifications from the perspective of the adjoining owner’s property and assess the structural risk that the works carry.

The engineer will then advise the adjoining owner’s Surveyor on the proposals, confirming if they are structurally sound, or alternatively if they can be amended or adjusted to reduce risk.

The engineer’s role isn’t to redesign the solution or pick fault in the building owner’s engineer’s design. Instead, they will often review the solution and point out areas where they believe the design could be improved or adjusted in such a manner to further reduce the potential risk to the adjoining owner’s property.

With the Advising engineer costs often starting at £500, this isn’t a figure to overlook.

Soil Investigation Reports

Soil Investigation Reports

Soil Investigation Reports are undertaken pre-construction work and will often see a borehole drilled on the building owner’s land likely to the same depth of the proposed foundation or base of the basement.

The outcome of the investigatory drill will be the identification of the exact fabric and makeup of the subterranean soil and will ensure an adequate foundation is designed taking account of the nature of the soil.

As you can see in the indicative image there are different bands of soil type, each with different characteristics depending on the location of the property and proposed depth of the excavation work.

With the Soil Investigation Reports costs often starting at £500 for desktop reports and £1,000 plus site investigation, this isn’t to be overlooked.

Cutting the Corbel Footing Cut Back and Underpinning the Existing Foundation

Cutting the Corbel Footing Cut Back and Underpinning the Existing Foundation

In order to accommodate the new subterranean basement rooms and extension, the existing Corbel Footing will need to be cut back, you can see this on the annotated plan.

The remaining footing supporting the structure and property above will then be underpinned. In practice, the foundation underpin will actually form the outer wall of the proposed basement extension.

Between the remaining footing and new basement wall, there will be a dry pack of slightly moistened portland cement mix. The process itself involves the dry mix literally being rammed into the newly formed cavity and ensures that there is a low shrinkage mix at the base of the existing structure thereby allowing the load to sufficiently spread to the new foundation or basement wall beneath.

In our experience, the dry pack is key, if it is too loose or too firm the structure above will move resulting in structural cracking and property damage. An experienced contractor should be able to get this step right and as a result, greatly reduce the risk of the issue.

Subsequent Use Expense

Subsequent Use Expense

Enclosure costs or subsequent use expenses are dealt with under Section 11(11) of the Party Wall etc Act 1996. It confirms that in the future if the adjoining owner were to undertake their own basement extension, they would then be making use of the newly built foundation or subterranean wall. By using this wall, the adjoining owner will not have to construct their own and therefore will be saving on construction costs.

Under the Party Wall etc Act 1996, the adjoining owner will need to reimburse the building owner for half of the cost of that wall.

This scenario will only come into play when the adjoining owner gains the benefit and use of the wall, however, it should offer the building owner some peace of mind as they may have some of the basement costs paid back to them albeit in the future!

Party Wall Surveyor Cost

Party Wall Surveyor Cost

Basement Extensions are by far one of the most highly complex areas of Party Wall Surveying and as a result, Surveyors are likely to spend a great more time on the file than they would for more conventional construction work.

There will also likely be the requirement for added visits both prior to the construction work commencing and during the course of the works, again adding to Surveyor’s costs.

Surveyor’s costs should be well considered before the works commence, from our experience we would expect a Party Wall Surveyor’s fees to start at £1,500.

We would recommend sending your proposed construction plans to our team of Surveyors as soon as they are ready. We will be able to give you a fixed cost quotation for the Surveying fees and also give you a firm indication of what an adjoining owner’s Party Wall Surveyor’s costs may be.

Party Wall Award Timing

Party Wall Award Timing

Agreeing a Party Wall Award for a proposed Basement Extension is a lengthy process and will require the input and discussion of both the building owner’s team and the adjoining owner’s Surveyor and his or her engineer.

In our experience there will likely be a number of design changes, circulating engineer questions and information requests, new drawings prepared and in some case site investigation.

These all add to the time it takes to agree to a Party Wall Award. In our experience, we would suggest budgeting in the region of 1 to 4 months from the point at which Party Wall Surveyors are appointed.

We would recommend taking this into account when planning a start date, the last thing you will want is the contractor raring to go, with Party Wall Awards still some way off being agreed.

Basement Extension Costs

Basement Extension Costs

Basement Extensions are by no means a low-cost form of construction. Depending on the type of property, the location and the proposed size of the extension, the costs are likely to be upwards of £250,000.

That being said with London Property Prices going up, the investment into the Basement Extension could be well worth it. In many cases, Basement Extensions can also be a great deal cheaper than selling your property and buying a larger one.

We think Basement Extensions are a fantastic way to get the most out of your home and expect these to become more and more common in the years to come.

Want to know more about Basement Extensions? Speak to our Surveyors…

We undertake numerous Basement Extension Party Wall Surveying jobs each year throughout London.

Over the years we have dealt with pretty much every type of issue imaginable on Basement Extensions and would be happy to discuss your proposed work.

Give our team of experienced Surveyors a call now.

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