THE ABSOLUTE BASEMENT GUIDE

Basements have the potential to significantly increase the value of your property, and your quality of living, adding additional floors of usable space to enjoy. Your new basement is a blank canvas and your design is not limited by internal walls. Over the years, we have built basements for kitchens, bedrooms, gyms, swimming pools, saunas, hammans, steam rooms, games rooms, cinemas, media areas, music studios, utility rooms, wine rooms, family rooms, plant rooms, the list could go on.

We’ve developed this guide from years of first-hand experience in the industry.  For anyone thinking about building a basement, this guide will put you in the driving seat, preparing you for each part of the process and giving you an extensive understanding of what’s involved. 

CAN I BUILD?

Basement builds are becoming increasingly popular in the capital, as a way to add space and value to your property.

If you are considering adding a basement, your first step is to find out if your plot or property is suitable. Although the majority of properties will be, ground conditions and restricted site access can mean that building a new basement isn’t always possible. Some reasons include unsuitable ground conditions, your property being in a preservation area or a listed building, a high-water table or archaeological finds.

We recommend that you discuss your basement with local estate agents to understand what your basement will add in terms of additional costs per square meter and ensure a good return on investment.

SO HOW DOES IT WORK?

Below is more in-depth look at each part of the process.  It will give you a greater understanding of basement construction, helping you to plan effectively and giving you the experience to hire the right people and ask the right questions.

PHASE 1 – PLANNING

Planning Application

Every borough in London has different policies for basement building. It’s important that you are aware of the latest basement policy and guidance of your local authority, so you know what kind of basement is feasible and any design or cost implications to consider.   If your house is in a conservation area there may be additional restrictions and costs that you will need to be aware of.

Surveys

The next stage will be employing surveyors to carry out the various surveys that need to be completed before the plans are submitted to your local authority.

These include:

• DRAINS SURVEY to understand how and where the drains run around and under your house. At this stage a build over agreement may be needed from your water utility provider.
• FLOOD RISK ASSESSMENT if applicable.
• GEO-TECHNICAL SURVEY  this is a soil investigation survey and will tell you the level of the water table as well as the type of ground conditions. Basement companies will require this to be able to give accurate costs and minimise any surprise costs.
• TOPIGRAPHICAL SURVEY or land survey to get an accurate representation of the area to show all natural and manmade features.
• ARBORICULTURAL SURVEY to examine trees to see what affect a basement build may have on any trees in your and/or your neighbour’s properties, and identify if any trees fall under a Tree Preservation Order.

Architects and plans

Appointing the right architect for the project is a major factor in getting it right!  A good architect with experience in basement builds will know the best ways of getting as much light into your basement as possible. These include front light wells, side light wells, rear light wells, stairwells and sunken terraces. Your architect will discuss your requirements with you, and with their knowledge of the local authority policies, they will be able to draw plans and you will see the project come to life.  You will see the size and shape of the basement, the room layouts, how the new basement will connect to your existing floors, where services (electrics, ventilation, gas etc) will be positioned and the shape it will be. It’s a vital part of the process to ensure you make the most our of your new space.

Informing your neighbours

Before the planning application is submitted, it’s a great time to let your neighbours know what you are planning.  Basement building will be disruptive to your neighbours, there are no two ways about it, but we’ve found that if you let them know what you are planning in advance, and what it will mean to you and your family, it usually helps.

You should assure them that you are using a reputable company to do the build, how long it will take, and show them the plans etc.  They will have concerns, which are understandable.  BaseSpace London management are always happy to attend meetings with you to explain the build in greater detail. It’s important that any concerns or questions are answered or dealt with as soon as possible, before planning.  This will avoid potential negative comments on the planning application portal.

Submitting the plans

The next stage is to submit the planning application.  Your architect usually does this, but you can do it on your own if you prefer.  Typically, planning will take around 8 weeks but it can take longer for more complex projects.  It’s very important that you try to understand what will be approved by your local authority before you submit plans as it is costly to make changes and resubmit plans.  Most planning departments do offer a free consultation clinic, which can be very useful before plans are submitted.

Once the local authority receives the plans, you will be assigned a planning officer, they may want to visit the property and ask further questions about the build.

Structural Engineers

Some local authorities require a structural design to be submitted with the planning application but some don’t.

If possible, it may be best to wait for the planning application to be approved with basic architects drawings and then start paying for structural engineers fees once you know you have all permissions in place.

It’s vital to appoint a structural engineer with experience in basement design. You may find an engineer who has cheap rates but if they have not got the relevant basement experience it may cost you significantly more in the build phase, if plans aren’t thought through and designed correctly.

The structural engineers will work on a series of drawings with very specific calculations. They will use the information from the surveys, and work with your appointed basement contractor to discuss any budget constrains, value engineering, time scales and any other factors to consider, such as site access. They will advise the best materials to use, underpinning/piling requirements and party wall detailing.

The structural engineer design is required before the Party Wall Agreement process starts.  The party wall surveyor will review the SE documentation as part of your Award.

 We can provide advice on the best structural engineers to use for every type of basement project.

Waterproofing design

This is a something you just have to get right!  As you can imagine, a leaking badly waterproofed basement is incredibly difficult to remedy once it is built so you must use a reputable waterproofing specialist to determine the best system to use at the design stage.

As Delta Membrane registered installers of waterproofing systems we can provide the best advice for your basement project.

Areas for consideration when waterproof planning are:

• Treatment of day joints and use of water bars during construction
• Post construction maintenance and flushing points
• Hydrostatic pressure implications
• Management of surface water / land drainage
• Sump pumps and their discharge paths
• Anti-lime treatments
• Flood testing post construction

As BaseSpace London are Delta Membrane registered installers we have vast experience in basement waterproofing. We treat the concrete structure with the most suitable products for the job to ensure that there is no ingress of water, prior to installing the membrane system.

If you chose to use a basement company who don’t have extensive knowledge in this area, we recommend using a waterproofing consultant to work as part of the design team.

Party wall agreement

You will need a party wall agreement if your property is within 3 metres  or in some cases 6 metres of a neighbour or boundary wall.  The Party Wall Agreement will advise your neighbours about the work and the date it is planned to begin.  The Statutory notice period prior to work commencing is 8 weeks.  Your neighbours have 14 days to respond to these notices and if they don’t, they are deemed to be in dispute of the proposed work and they are then obliged to appoint a party wall surveyor and these costs may be passed on to you.  The surveyor will work with both parties to be able to award a party wall agreement.  Only when you are in receipt of the agreement, can you start the work.

Appointing a basement contractor

At BaseSpace London we recommend that you plan your basement in two parts, the basement shell and core and the internal design and fit out.

A basement specialist will create the waterproof concrete structure in the ground. They specialise in this and can offer the best, guaranteed waterproof structural shell.

The fit-out specialists will then take the concrete shell and make it look pretty. You can choose a fit-out team that’s perfect for your needs, and your budget. They will add the plasterboard, plastering, electrics, plumbing, flooring, joinery, kitchens, bathrooms etc and make it ready for you to move in to.

The most efficient and cost effective way of constructing your basement is to get your shell and core contracted first.  The basement contractors will need the design, structural drawings, calculations and scope of work for them to price the job.

BaseSpace London can offer a budget cost for your proposed works without all the design information. This way you can see if it is a viable option financially for you to construct your basement prior to spending out on too many design fees and applications.

Basements are a complex and specialist type of construction and you must employ a contractor with vast, verified basement experience. It simply isn’t worth taking the risk with a cheaper, less experienced contractor.  As the trend for basements increases, more and more builders are adding basement to their offering but unfortunately very few have the technical know-how, experienced teams, and credentials to do the work and provide the required guarantees.

It is very important to get references and not to rely on savvy salespeople and fancy shots of finished basements. You should visit your short-listed contractor construction sites in progress to see how they work. If the site looks a mess and the health and safety is not good, the works are likely to be sub-standard too.

Once you have chosen your contractor make sure that you have a clear scope of work and agreed fixed costs with your basement contractor.  Unless your scope of work or designs changes the cost should not increase once the costs are agreed.

PHASE 2 – STRUCTURAL SHELL

Structural Shell Steps

• Hoarding to be erected in the front of property and skip positioned (front garden or road in front of property)
• Excavation of the front light well and conveyor installed (to remove earth)
• Underpinning (1m section at a time)
• Install steel beams
• Excavate other lightwells/stairwells
• Excavate central mass once underpinning is complete
• All drainage to be installed and delta pumps
• Pour concrete slab (basement floor)
• Waterproof basement using Delta membrane
• Insulation installed on top of membrane
• Under floor heating installed (if required)
• Floor screed to be laid

Inspections

Once the build begins, an approved Building Control Inspector will need to be appointed to oversee the construction work and sign off that the structure conforms to Building Regulations. This will require visits at key stages, which will be co-ordinated by your basement contractor.

PHASE 3 – FIT-OUT

Once your basement excavation is underway, it’s time to get quotes from fit out contractors.  You will need to send the package of drawings and schedule to enable them to quote accurately.  We recommend asking around for recommendations, getting at least 3 quotes from different companies and visiting them mid project, so you can see how they work, speak to previous clients and visit finished work as much as possible.

BaseSpace London have preferred partners that we have worked with in the past to suit every budget. We are more than happy to help you decide the best contractor as this is a very important part of the build.

INTERIOR DESIGN

If you are using an interior designer for your project, ensure that you follow the same rules as selecting basement and fit out contractors.  Ask for recommendations, visit previous clients properties.   Once you have selected the Interior Designer, set up a meeting with the fit-out company and discuss the plans in full.  You need to ensure that your basement interior design is well thought through, it’s the icing on the cake to make your new space the best it can be.