The London Guide to Under Garden Basements

We've created a handy guide for those with a big garden or driveway who are thinking about a house extension. Read online or download the PDF and discover why we recommend an under garden basement or under driveway basement.
 |  Architecture

For those with a big garden or driveway who are thinking about a house extension, we recommend you consider an under garden basement or under driveway basement. 

Dig deep to get a high ceiling, put in light wells and roof lights to get an incredible space that feels just as good, if not better than your accommodation upstairs.

These are some of the rooms and types of accommodation that we are seeing under garden basements being used for: 

  • cinema room
  • kids playroom
  • games room
  • wine cellar and wine tasting room
  • cigar room / walk-in humidor
  • gym
  • sauna
  • swimming pool
  • spa
  • massage room
  • sanctuary
  • nanny, granny and teenage flats
  • bar
  • nightclub dance floor
  • bowling alley
  • recording studio
  • luxury car park

Interested?

This guide will explain how Ensoul Architectural Design can take you all the way from initial interest, through the planning application process and then into construction and handover.

Step by step guide

What follows is a comprehensive, step-by-step guide to under garden basements from first concept through to final handover back to you as the client. This same sequence equally applies to under-house as well as under-garden basements although some of the emphasis/detail will be different in some of the steps.


Under garden basements add significant value to your property; Ensoul has delivered more than 200% return on investment for under garden basements in terms of the uplift in property valuation versus basement cost for several clients. But aside from adding serious amounts of floor space and value, here are 5 great reasons to go under your garden instead, or as well as under your house.

An under garden basement in London with lightwells and terrace

1. Brighter and lighter

By building a basement under a garden or driveway, we can architect in walk-on roof lights, lightwells and terraces to flood your basement with light.

2. Fresher

With lightwells and lower terraces you can throw open the doors to bring in fresh air and, with some planting and furniture you can even enjoy a small subterranean garden.

3. Quicker

If you have good side access to your garden, installing a conveyor belt is much easier and could save you considerable time and hassle on your construction.

4. Bigger

If you are fortunate enough to have a long or wide garden, you can create a large basement footprint as an alternative or in addition to an under house basement.

5. Lower impact to you and your neighbours

Under garden basements are less of a structural risk to both your property and your neighbours' properties as they are not underneath. They also create less noise and movement during the construction period as you are further away from your houses.


Step 1: Understanding the planning rules

Planning restrictions

Every planning office has a different policy regarding what you can and can’t do with under garden basements and basements generally in their borough. It’s therefore important that you investigate the latest basement policy and guidance of your local authority at the very start so you know what kind of basement is feasible, the requirements attached to the application and any design or cost implications. For example:

  • Wandsworth basement policy recommends that it would be good practice to retain a depth of at least 50% of the back garden.
     
  • Merton basement policy stipulates not to exceed 50% of either the front, rear or side garden of the property.
     
  • Royal Borough of Kensington & Chelsea basement policy limits the extent of basement excavation to no more than half the garden and the depth of a single storey only.
     
  • And Westminster’s new basement policy has introduced a levy from 1 September 2016 for a subterranean squad of “basement nuisance busters’ – charged with overseeing and controlling traffic impact, stricter working hours and complaints. It’s the first of its kind in the country and assumed to be picked up by other boroughs eventually.

Conservation Area restrictions

If this is applicable, you will need to ascertain any additional restrictions placed on a basement development in your area.

Step 2: Ensuring Value

You need to conduct desk research and talk with local estate agents to establish what your newly proposed basement will add in terms of additional pounds per square metre. You can then ensure that the cost to value ratio is in balance and that you get a great return on investment.

We have seen the ratio of property value per square metre to build cost per square metre at 2:1 or more in some cases.

Step 3: Surveys

Before you start to think about the design and planning application, you will need to get surveys done to inform the basement technical design and possibly to be included in your application:

  • Geo-technical survey - this reports on the soil structure down to the depth you are digging and any likely issues regarding the height of the water table.
     
  • Drains’ survey to ascertain where the drains run around/under your house and garden to understand the impact of the basement construction. Depending on where the drains run, a separate “build-over” agreement may be required from your water utility provider.
     
  • Topographical survey to get accurate dimensions of the site.
     
  • Arboricultural survey to investigate trees in yours and your neighbours’ gardens to see what impact a basement dig may have on the health of any trees. Also, as part of this step, identify any that fall under a Tree Preservation Order.

Step 4: Architectural design & space planning

Once armed with local authority policy, the necessary survey results and an outline budget estimate, you will need to design your basement space plan which will include:

  • How big and what shape it will be
     
  • How you propose to connect your new basement to your existing house
     
  • What’s the best space plan and room lay-out
     
  • A high level interior design plan and furniture lay- out so that you know the architectural design is in perfect harmony with the interior plan
     
  • Where to put services i.e. drains, electric runs, ventilation, etc.
     
  • Access and exit points including compliance with building regulations
     
  • Roof lights and light wells to provide lots of natural light and ensure a high quality living space
     
  • A garden lay-out that will go back on top of your new basement

Step 5: Neighbour Support

At this stage and definitely pre-planning application, we recommend making the time to book 1:1 meetings with your close neighbours to talk about your plans over a coffee or a glass of wine. Ensoul can help to create a presentation pack for you and can attend if it’s helpful to answer technical and build questions.

The main things to communicate are:

  • Explain why you’re wanting to create your new space and what it will mean to you / your family. Neighbours are far more supportive if they understand that you need more space for music practice with contained noise levels, or space for a live-in nanny so that you can go back to work, etc.
     
  • Provide them with a top-level overview of the proposed basement size and design.
     
  • Go through timings with them so they can.
     
  • understand the duration of works and impact to them.
     
  • Explain that it is being professionally designed, built and managed to quash any fears about “cowboys” potentially collapsing your houses.
     
  • Make a small gesture to them e.g. pay for a window cleaner once a month during works. The goodwill will go a long way and in the scheme of things, the cost is minimal.
     
  • Critically, get any concerns or questions answered there and then, or written down so that they are dealt with immediately or if not there and then, before planning. This will prevent potential complaints or open comments on your local planning website when you file your application.

Step 6: Planning Application

Next, finalise your plans, write the necessary content and file your planning application which will take up to 8 weeks from acknowledged receipt by your local planning office.

Typically, your assigned planning officer will want to visit and will have some questions that need answering and are likely to indicate some conditions that may be required to give planning consent. Ensoul can handle all of this on your behalf through to conclusion and result.

Step 7: Technical and structural design

Structural design and calculations:

A structural engineer now needs to be appointed to decide and detail the best way to design, support and build your basement shell and core. The ‘shell and core’ is fundamentally the concrete and reinforced steel structure in the ground. The engineer’s design will factor in all survey information available as well as your project timescales and budget constraints. The resulting piece of work is a series of drawings and calculations which identify the construction sequencing, materials, steelwork, under-pinning and/or piling details, party wall detailing and CDM (Construction Design & Management) requirements. The structural design is required before the Party Wall Agreement process starts as any appointed surveyor will want to review these as part of your Award.

Waterproofing design:

How the basement structure is designed will also determine the best waterproofing system design. On the assumption that your basement is structurally sound, waterproofing it is the next most important aspect of your build. A leaking basement creates enormous stress and is an incredibly difficult problem to solve. Because water travels, it can be very difficult and in some cases, impossible to find the entry point. So, you must ensure you have the very best waterproofing experts designing your basement from the start. The way the waterproofing system is designed also has implications for the insurances/guarantees you will need to put in place. 

Some of the larger specialist basement construction companies have extensive skills in this domain in-house. However, it is a highly specialist and skilled area and when using smaller firms, we recommend engaging an experienced waterproofing design consultant to work as part of the design team. If you would like to find one, here’s a great place start your search: http://www.thebswa.plus.com/ 

Architectural design:

Based on the Planning Application drawings, the next level of detail can now be developed.

Step 8: Engaging a basement contractor to build your basement

We recommend that you think about your basement works as two distinct projects:

  • Basement shell and core: this is the creation of your waterproofed, concrete structure in the ground. A specialist contractor, as opposed to a general building contractor, is required for this basement construction stage. Make sure that you are contracting the actual contractor that is going to conduct these works – not a firm that is going to subcontract these specialist works and simply put margin on it.
  • Basement fit-out: this element is focused around the interior finishes and includes for example, wall formation, plastering, flooring, lighting and electrics, plumbing, decoration, joinery, etc. Most basement contractors only focus on delivering shell and core packages and are not interested, or sufficiently experienced, in delivering fit-out work.

The most efficient way of constructing your basement is to get your Shell and Core contracted first as a detailed package of works with a fixed price. This way, you can get your project up and running and whilst your basement is being dug out, you can be designing the interior of your basement (either yourself or with a professional), and selecting all the internal finishes. Once this work is done, you can get an accurate scope and budget put together by your design firm and/or fit-out contractors. 

There are a small number of high quality building firms that can take on both the “shell and core” and the fit out, however, predominantly, they are different companies with very different skills.

Contracting the shell and core:

Once in receipt of the architectural plans and structural engineer’s drawings and calculations, you will need a detailed scope of works ready to discuss with pre-vetted basement contractors. There are two main types of contract we use:

  • JCT Design and Build Contract (watertight shell and core)
    This provides a single point of accountability for all elements of your basement shell and core design and build including:
    • Structural and temporary works design
    • Waterproofing design and drainage
    • Basement shell and core build
    • External glazing
       
  • JCT Intermediate with CDP (Contractors Design Portion) and minor works contracts
    ​​​​​​​These contracts are typically used when it makes sense to break out one or more elements of the basement design and/or build to specialist 3rd parties. This decision may be taken if the preferred basement contractor has a lack of expertise / experience with a particular element, or because there is a significant cost variation between having it all under one roof vs taking some elements out of scope. For instance:
    • The structural design could be contracted  with an independent structural engineer 
    • The waterproofing installation could be contracted with a specialist waterproofing professional. 

Guarantees and insurances:

We strongly advise that you take expert advice on guarantees and insurances. 

If the unthinkable happens, your basement leaks, and your contactor has gone bust, what happens next? Who are you going to try and claim against? Or, if you decide to sell the property and the buyer asks what guarantees are in place, how are you going to give their conveyancing solicitors comfort that all the guarantees are robust.

  • The fundamental elements to consider include:
  • Builder company guarantees
  • Waterproofing guarantees (design, product and installer)
  • Insurance backed guarantees
  • Latent defects insurance
  • Building’s renovations insurance (Joint Names)

Inspections

During the build, 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.

Excavation and building the concrete box

Now the works can start and the next phase begins which is managing the basement contractor on-site to deliver against the design details that have been developed without variation. This deserves an article in its own right rather than describing the extensive detail here.

Step 9: Interior Design

If you have not done your interior design beforehand, you can do so once your basement contractor starts works on the shell and core as this element will take a few months before completion.

Your basement interior will need designing and all fixtures and fittings will need specifying e.g. roof lights, light wells, windows, external and internal doors, staircase, floor and wall finishes, sanitary ware, tiles, paint colours, AV and IT networks, HVAC, heating, ventilation, air conditioning), security, light fittings, furniture selections, etc.

For a beautiful result and efficient flow of works, hire an interior design company to put together a cohesive scheme and to recommend full product selections to you.

You would be amazed at how many products and finishes you will need to pick and agree, so if you can afford to bring in help, we recommend you do so.

If you’re bringing in a designer, make sure they can handle the technical design such as lighting plans, floor make-ups, glazing details, etc. as well as the ‘pretty stuff’. It also pays to ensure that your design team can deliver a technical CAD package of drawings and detailed schedules so that you can see exactly what you are getting and sign it off; and so that the builder knows exactly what to cost and then build to with no ambiguity.

Step 10: Fit Out Contract

Once you have your agreed fit-out package of drawings and schedules, it can be sent out to fit-out contractors to provide a fixed price quote. You or your designer will also need to get all of the products quoted as well by all the various suppliers once you once you have chosen everything to create the fixed price budget.

By selecting everything before the fit-out starts, you can make sure that your basement interior design is well thought through and cohesive and that you are getting an accurate fixed price quote from builders with everything detailed and included.

It also means that you can fix a price at the start with a contractor and ensure that when they start works, they have all the information they need to deliver the programme of works.


Article summary and conclusions

As you will see from this article, there is a considerable amount of detail to be managed, third party service providers to co-ordinate and tendering processes to go through. Think seriously about getting an independent Project Manager with experience in basement construction. We’ve written an article on the benefits of professional project management and defined the key areas of responsibility / scope of works that one should be contracted to deliver against.

One of the common questions we get asked is “How long will my basement take to build once contractors start work?” For an average basement of say, 100sqm and one storey deep, allow approximately 9-11 months which breaks down broadly as follows:

  • Watertight shell and core 5-7 months:
    • Site set up
    • Excavation, temporary works, dewatering, underpinning/piling
    • Floor slab and sump pumps
    • Staircase and roof slab
    • Waterproofing and drainage
    • Underfloor heating and screeding
    • Rooflights and lightwell glazing
    • Testing & certification
    • Site demobilisation
  • Fit-out: depends on complexity of build/finishes but allow 3-4 months
  • Landscaping (if it’s an under garden): depends on complexity of design but allow 1 month (this can occur in parallel with the interior fit-out)
  • Furniture & soft furnishings: subject to order dates and lead times

In conclusion, under garden basements add a fantastic amount of living space and can significantly improve your quality of life. Our advice to anyone embarking on a basement project is to:

  • Engage an experienced design team at the concept stage to ensure you optimise the space design, room lay-outs and quality of light to deliver a well designed, high quality living space.
  • Ensure that you bring in experienced and capable contractors who know how to safely construct basements and make sure they’re watertight. Beware of going cheap.
  • Bring in expert knowledge to write and package up your contracts with the builders and to make sure that you have the right insurances, guarantees and warranties in place.
  • Seriously consider getting a professional project manager who is experienced in basement builds to manage all contractors and third party professional firms from the initial design stage right through to hand-over.

Want to find out more or get started?
Call us today on 020 3637 0700 or email transform@ensoul.co.uk 

Download the PDF
Download or read The London Guide to Under Garden Basements and learn how we'll work with you from concept to planning approval.

 

An under garden basement by Ensoul

 

Author

Mike Lander - Residential Project Manager, Programme Director
Project Manager & Programme Director
Mike is a highly experienced programme director and engineer and runs Ensoul's project management and smart home operations with unrivalled precision.

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Architecture