Rear extensions 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 and also dramatically increase the kitchen.
Alongside planning requirements and building regulations, the Party Wall etc Act 1996 will also need to be considered and adhered to. While a rear extension is a very conventional form of residential construction work, as with any construction it carries risk, part of mitigating that risk is going through the necessary Party Wall Surveying procedures.
We thought we would take a look at Rear Extensions through a Party Wall Surveying perspective in the hope of helping you understand the type of considerations that Party Wall Surveyors will look if they act for you or for your neighbour.
Building a Wall up to the Boundary Line
If the proposed flank wall of the rear extension is positioned up to the boundary line, known as the “line of junction” under the Party Wall etc Act 1996, then the building owner (the owner undertaking the works), gains the legal right of access onto the adjoining owner’s land (the owner adjoining the works). The access will agreed via a Party Wall Award and will ensure that it is subject to adequate protections, while also ensuring the duration of the access is reasonable. You can read a little more about Party Wall Access below.
The location of the proposed flank wall will have a bearing on the Party Wall Notices that need to be served in advance of the proposed works commencing, therefore it is imperative to have a clear idea of that sooner than later.
One of the most common misunderstandings that owners have is that the flank walls of the proposed extension needs to placed at a distance from the boundary line, thereby leaving a gap for future maintenance. This is not the case, a building owner has the right to building within the full curtilage of their land.
Building a Wall Astride the Boundary Line
If the building owner wants to build the flank wall of their extension astride the boundary line, they will effectively be building a new Party Wall half on their land and half on the land of their adjoining owner.
This type of construction can only occur if the adjoining owner is agreeable to a Party Wall being built, or if there is already an existing shared garden wall, known as a Party Fence Wall separating the two owners’ gardens.
The benefits of a Party Wall built astride the boundary line is that in the short term the building owner will benefit from a slightly larger and wider extension. However, it is worth noting that as the wall is a Party Wall, if the adjoining owner plans to undertake their own extension in the future, they will have the right to build directly off the wall, again leading to a larger extension for them.
This right is subject to a making use, or enclosure payment from the adjoining owner to the building owner which is effectively 50% of the cost of the wall at the time of enclosure.
As with a wall built up to the line of junction, a wall built astride the line of junction, also affords the building owner access rights onto an adjoining owner’s land. You can read a little more about Party Wall Access below.
Concentric Foundation
Concentric Foundations are foundations that equally distribute the structural load of the wall and structure to the left and right of the foundation. This type of foundation will also mean that there is a projecting outer face, or edge to the foundation that will thereby limit the location of the wall above.
If the building owner wanted to have this type of foundation for a proposed wall that was built up to the line of junction, that would mean that the projecting outer face, or edge to the foundation would be placed on the adjoining owner’s land and could prejudice their future development rights or overcomplicate their future construction designs, as the projecting edge of the foundation would need to be taken into account.
While the building owner does have the right to have their foundation on an adjoining owner’s land, that right only exists if it is “necessary”. With engineering being at the point it is today, it is almost always possible to design around this need, therefore it is very rare for a Party Wall Award to give the building owner right to place foundations on an adjoining owner’s property.
Eccentric Foundation
Eccentric Foundations are foundations that allow the wall and structure that they support to sit on the outer face of the foundation.
The benefit of this type of foundation is that it allows a building owner to build within the full curtilage of their land without the need for any part of the foundation to be placed upon the land of the adjoining owner.
This ensures that the building owner’s foundations do not detrimentally affect the adjoining owner’s future development rights, as the adjoining owner will not need to design around any projecting foundations.
Surprisingly, the use of eccentric foundations is actually rarely designed into the building owner’s proposals, therefore it is imperative to ensure that these are implemented and importantly placed wholly on the building owner’s land. A Party Wall Award will ensure this is the case.
Build Over Drainage
The majority of semi-detached or terraced properties within the UK will share subterranean drainage pipes, which will usually run parallel to the rear walls of the houses.
In many cases the pipes and drainage will service both rainwater and foul waste (toilets, kitchens etc) and will run to the main sewer which services the street where the properties are located.
The location of the drainage pipes will often mean that the proposed rear extension’s foundations will need to be built over or around the drainage pipes, as they are effectively in the way of the proposed foundations.
While Thames Water will require the building owner to obtain a “build over agreement”, the Party Wall Surveyor will also want to ensure that the drains are taken into account and that the necessary foundation detail is available. The Party Wall Award will ensure this drawing is prepared and on file.
In some cases, a CCTV drain survey or investigation will also be necessary in advance of the construction works. The survey ensures the condition of the subterranean drainage pipes is recorded pre and post works thereby ensuring that the building owner’s Party Wall works do not cause damage to them.
A Flashing Drawing
If the adjoining owner has their own rear extension built up to, or near the line of junction and the building owner is planning their own extension alongside that. The placement of the two flank walls will likely mean there is a small gap between them that will need to be considered and weathered against to stop penetrating damp.
This weathering solution is commonly achieved by a flashing drawing or detail. The drawing confirms how the flashing will be placed and what the fabric of it is.
Commonly the flashing will be code 4 lead (1.8mm thick) and will be dressed into, or over the adjoining owner’s flank wall thereby ensuring a long lasting weatherproofing solution.
Surprisingly, weatherproofing and flashing details are rarely designed into the building owner’s proposals, therefore it is imperative to ensure that these are implemented to safeguard against future damp issues. A Party Wall Award will ensure this is the case.
Sectional Hit and Miss Bays
If the adjoining owner has their own rear extension built up to, or near the line of junction and the building owner is planning their own extension alongside that. The location of the building owner’s proposed foundations will mean that if excavated and dug in one go, the adjoining owner’s extension would be left unsupported which could be prone to movement.
In order to avoid this, the Party Wall Award will specify that the building owner’s proposed foundations are dug in sequential hit and miss bays which are usually no more than 1m in width.
These bays ensure that the adjoining owner’s foundations are always supported which greatly reduces the risk of movement and subsequent property damage.
In most cases, the contractor will not want to dig in this manner as it is more time consuming that digging in one go. While it can take longer on site, the reduction in risk is well worth the temporary delay and as such this is a common clause within Party Wall Awards.
A Foundation Slip Membrane
If the adjoining owner has their own rear extension built up to, or near the line of junction and the building owner is planning their own extension alongside that. The location of the building owner’s proposed foundations will mean that there is the risk of the two respective foundations becoming toothed together which can cause structural issue further down the line.
In order to avoid this, the Party Wall Award will specify that the building owner has to install a slip membrane alongside the adjoining owner’s foundations. This ensures that the two foundations remain independent of one and other and move and settle without one affecting the other.
Foundation slip membranes are a simple on site solution, however in many cases unless the contractor is instructed to install them they tend not to.
Party Wall Access
Under the Party Wall etc Act 1996, if the building owner’s proposed extension’s flank wall is built up to the line of junction, the building owner is afforded the legal right of access via a Party Wall Award onto the adjoining owner’s land in order to construct the wall in a safe and timely manner.
The access will be specified and agreed by the Party Wall Surveyor, who will ensure that it takes into account all of the adjoining owner’s reasonable concerns, the lay of the land and the proposed works.
The access will also only be a for an agreed period of time, which is commonly the necessary time for the building owner’s contractor to construct the flank wall of the proposed extension.
The area where access is required will be fully screened off, meaning that the inconvenience and nuisance to the adjoining owner is kept to an absolute minimum, or as close to a minimum as it reasonably can be.
Party Wall Award Timings
In our experience agreeing a Party Wall Award for a proposed Rear Extension can be a relatively straightforward process, however there can be delays around the Party Wall Notice response stage and also around the timings of arranging a pre works Schedule of Condition Report of the adjoining owner’s property.
We would suggest budgeting in the region of 1 to 2 months from the point at which Party Wall Notices are served.
We 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.
This timescale would also assume that all of the necessary information is in place we have touched on some of these within this guide.
Rear Extension Costs
Rear 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 £50,000.
That being said with London Property Prices going up, the investment into the added space achieved by Rear Extension could be well worth it.
We think Rear Extensions are a fantastic way to get the most out of your home and ensure that you are making the most of its potential.
What to know more or speak to one of our Surveyors?
We undertake numerous Rear Extension Party Wall Surveying jobs each year throughout London.
Over the years we have dealt with pretty much every type of issue imaginable on Rear Extension jobs and would be happy to discuss your proposed work. Give our team of experienced Surveyors a call now.
