Successful basement project: an Architect’s advice 2023
Undertaking a basement is a serious project and involves various important factors, from hiring an architect and designing the space, navigating party wall issues, obtaining planning consents and carrying out the build itself. This article has been written to shed light on the process and point out what to watch to have a successful project along with time scales and costs.
It’s crucial to acknowledge that subterranean spaces may not be ideal for kitchen or dining areas due to limited natural light and views. However, basements can serve as excellent spaces for informal family areas or entertaining spaces, such as cinemas, playrooms, home gyms or swimming pools.
Bloom associates in partnership with Malone + Pike have a strong track record of securing consents for basement projects across different areas of London, including Wandsworth, Clapham, Fulham, Chelsea, Lambeth and Southwark. Feel free to explore our recent work portfolio or contact us at 020 7 585 2020 to discuss your project.
Determining Suitability for a Basement Extension
Several key initial considerations should be evaluated from the very beginning:
- Construction Costs: Work with your architect to estimate the construction expenses. On average, a finished basement costs approximately £5,000 per square meter, excluding VAT and fees. Costings for basement shell is quite linear and simple; the bigger / the deeper, the more expensive if will be. I suggest you should look to achieve a finished head height of over 2.5m to make the space feel as “above ground” as possible. For more info on costs head to our Process & Fees page.
- Value Added: Assess whether the potential value added to your house justifies the construction cost. Consult local estate agents and present your plans to estimate the value uplift. In Clapham, windowless basement space is currently selling for around £7,500 ex vat (Jun 2023). This means that basements in Chelsea make sense but may not make financial sense in areas where the price per square foot is lower.
Selecting an Architect for Your Basement Extension
When choosing an architect for your project, prioritise those with experience in basement projects. They will be well-versed in local planning policies and possess the technical expertise required for these complex projects. An architect will guide you through the entire process, including budget assessment, design development, planning consents, builder selection, and construction support.
Securing Planning Permission for a Basement
The local authority is only concerned with the external envelope of the proposed works. With basements, this usually means lightwells to the front and rear elevations. Consulting with an architect can provide more guidance tailored to your specific situation. In theory, obtaining consent for basement applications should be relatively straightforward, as the alterations are mostly below ground level and less visible than other types of extensions. The one constraint to consider is that 50% of the front and rear gardens must remain un-excavated. This prescribes what you can do with the front lightwell and if you have a small front garden area it will limit what you can do with the front lightwell. Railings around lightwells tend to be resisted in planning with flush floor level metal grills or glazing being preferred. The benefit of metal grills is that is allows both light and air into the basement.
The main planning criteria are:
- Basements must not exceed the property footprint, plus a maximum of 50% of the garden.
- They must not be more than one storey.
How long does it take?
It takes us approximately 4 weeks to put together a drawing pack that is ready to be submitted to the council for approval. This time scale is dependent on how many design revisions are required to reach an agreed document ready for submission. Once submitted, it should take the council 2 months to respond.
It should take 3 months to obtain planning. permission
A basement is usually a “householder application” which currently costs £206 and is paid directly to the local authority by the applicant.
The detailed design and construction drawings take around 10 weeks to complete and can be carried out during the planning application. This includes a 4 week period for the structural engineering to be carried out. The structural engineers job is to design the structural elements (floor, underpins, retaining walls and steels etc to make sure that the architecturally designed structure stands up. The structural engineer’s design is also required for the party wall notices to be served. If time is of the essence, we suggest that the engineering is started ASAP to allow for the party wall notices to be issued as soon as possible.
Once planning is approved and party wall awards are consented works can start on site. The duration of the build depends on the size and depth of the basement combined with site conditions. Typically, half basements (around 40 sqm) take around 6 months to complete. Full basements (around 80 sqm) take around 12 months to complete.
Party wall process
The party wall process can be complicated with basement projects. Understandably, neighbours may not be thrilled by your plans to carry out a basement project because it is major structural work to shared party walls, noisy and disruptive. You have a legal responsibility to follow the party wall process if you are underpinning party walls and should anticipate the process to take a while and be challenging. A typical party wall process will include:
- Serving notices including the structural engineering details of the foundations.
- Having those engineering details checked over by a third party (checking engineer) (at your cost) and challenged.
- Have various method statements drawn up for approval by the checking engineer and subject to revisions as they see fit.
- Pay enclosement costs if your project benefits from any previous work carried out by the adjoining owner.
- Put money in Escrow for the event that the construction stops part way through the work. The cost of this Escrow will be agreed by the party wall surveyors but I have seen it run to £20,000+ per neighbour. This money is released upon completion of the structural works and subsequent inspection by the party wall surveyor.
- Carrying out a “schedule of condition” on the neighbouring properties to record any existing damage or defects in the existing property.
The party wall process is the bit of the project which I personally find the most challenging as it is outside of everyone’s control. We suggest that a neighbourly informal approach, keeping your neighbours informed on your plans and sharing the drawings with them prior to submission, offering to have a schedule of condition and expressing you understand their concerns is the best way to have a smooth process.
Light and air
- Get as much light as possible into your basement with big, floor to ceiling windows to your lightwell. It is very difficult to retrofit bigger windows or larger lightwells so get it right at the design phase and don’t skimp on this.
- Consider passive ducted ventilation to internal windowless rooms. There are more sophisticated air management systems for basements that mechanically circulate and even filter air which is becoming increasingly popular but as with any mechanical technology, it comes with maintenance and servicing commitments.
- Establishing visual connection between the basement and the rest of the property will help it feel part of the main house rather than a bolted on accessory. I consider the staircase and landing area leading to the basement to be the foundation to a integrated basement. It needs to feel like an extension of the original staircase rather than a poky steep narrow stair.
- Put back stage stuff in the dark central parts of the basement and keep the light rooms to the everyday spaces. For example, utility, wine cellars etc do not need any natural light but a bedroom does.
Heating and cooling
- Basements are largely unaffected by external temperatures as the temperature below ground tends to be around 24 degrees centigrade summer or winter. However, the connections into the basement and lightwells etc will allow the heat from the air to warm or cool the basement space.
- As the engineering will likely involve a concrete slab foundation, it is a perfect opportunity to install underfloor heating pipes to heat the space which is a low cost effective way to provide heating.
- If you are considering a gym, wine room etc, you may benefit from adding in cooling as well. The external unit can be hidden within the lightwell or elsewhere on the property.
- Waterproofing is achieved using a membrane system (Delta or Newton are reputable suppliers) which is fixed to the walls and floor of the basement shell. The membrane allows water to enter into the structure where it is then diverted into sump chambers and pumped out to the mains drains. The sumps often come with alarms and battery back ups to make sure they work at all times.
Full basement or half basement
- Many people decide to only do a half basement under the two front reception rooms because it is simple to excavate under the suspended timber flooring generally found in the front sitting rooms of victorian properties. If you decide to go under the kitchen, you need to change the floor to a suspended floor from a solid floor so you can dig under it. The kitchen, flooring and structure will have to be removed and changed before works can happen. Digging under the kitchen is more complicated and expensive than sticking to the front only.
Licences, conveyors, parking bay suspensions and skips
Prelims on basement projects are an enormous consideration as they can be very expensive an onerous to sort out. Kensington and Chelsea council charges up to £132 per day for a 5m parking bay suspension. You also need skip licences, hoarding licences, converyor licences and all sorts of consents from the council. When you consider that some times we will need a suspended bay for 6 months, the costs add up and it is not uncommon to spend £20,000 + on fees to the council when carrying out basement projects.
On an 80 sqm basement, you will need over 60 skips to remove the soil. Skips hire alone costs £300 each and you will need to budget a further £20,000.
Basements are the biggest project you can take on as a residential property developer. They are lengthy, expensive, complicated, neighbours hate them and the local authority will make a fortune in fees for licences. We recommend that you consider basements as a last resort for adding space to your property as building above ground makes so much more sense on all fronts. When you have decided to build down, do it with conviction and build it properly first time with good head height, maximum light and air and make the most of your new space.
Bloom and Malone + Pike are ready to help with your basement plans.