How deep should a basement lightwell be?

[Strak]

Hi there, just joined the forum and hoping to get some advice for the extension I'm currently planning.

 

I’m having some plans drawn up for a ground floor extension, with a basement built underneath. We're comparing the basement option with a loft conversion, the main advantage of the basement being that it would give us more space and be cooler in the summer. The main disadvantages are of course cost and light.

 

The basement will have 2 bedrooms and a bathroom with a staircase to get upstairs to the living room and kitchen area, and a shared light well for the two bedrooms with an external staircase to get into the garden.

 

I’m struggling to know how deep the light well needs to be to let in enough light to make the two bedrooms a pleasant space. Currently on the designs it is 1800mm deep, spanning the width of the house. However, I would like to make the ground floor extension bigger without taking much more out of our back garden, so I'm thinking of making the lightwell smaller and adding some adjustable LED daylight panels (Lindby Kjetil LED ceiling panel 120 x 30 cm | Lights.co.uk) into the basement bedrooms.

 

I have attached the first design draft, although internal things will move it gives a good idea of what we’re looking at. On the front/rear elevation page you can also see the loft conversion which we are considering. Some changes of note that we will make compared to this first draft: basement will be under the living area, dining area will be removed to create a bigger utility and we want to make the living area a little bigger to comfortably have a dining table in the living area.

 

Does anyone have some experience of how deep a light well should be to let in a nice amount of light? Would the LED panels on daylight setting be good enough to compliment the natural light to make the basement rooms feel light? The rear garden (and therefore light well) are south facing. Also considering adding a shower en suite to one of the bedrooms by using the end of the light well which would reduce the size even further (approx 1.2m on one side).

 

plans.pdf

 

 

Thanks!

[tonyshouse]

Light wells don’t need to be anything like that deep, normal window 10% of floor area, for high level windows less area is needed. 
 

where is ventilation coming from? 

[Strak]

@tonyshouse thanks for your reply. So based on what you said it means that I could quite comfortably reduce the light well to closer to 1 metre wide, assuming that I could fit the stairs in?

 

I assumed that the ventilation would be from the light well too, and there will also be a corridor and staircase into the main building too. Is there a need for more ventilation than that?

[George]

It's not a direct answer to your question, but a new build basement under an existing building is not a good idea. Difficult, risky and expensive. The only places that do it are where the land values are super high (i.e. central London) and even then, they can go very badly wrong. Basement conversion is OK but still face challenges with waterproofing. 

 

You'd be better off doing the loft conversion and using the money saved (which would be a LOT) to put in solar panels and an air conditioning system. 

[markc]

11 hours ago, Strak said:

@tonyshouse thanks for your reply. So based on what you said it means that I could quite comfortably reduce the light well to closer to 1 metre wide, assuming that I could fit the stairs in?

 

I assumed that the ventilation would be from the light well too, and there will also be a corridor and staircase into the main building too. Is there a need for more ventilation than that?

Looks like you are confusing depth with area, or are you referring to depth on plan? as opposed to vertical depth?

A larger area light well will allow more light in for more hours. Light wells are complex to get right, you need to look at orientation, shadow from other buildings, trees, fences etc.

[Dudda]

Ignoring the cost as requested which is my biggest concern, it's a balancing act. You could get a daylight analysis study done. It's a 3D version of the house done by an environmental engineer and measures the daylight available. We get them done a lot (for larger projects not houses) and they're very useful. We always get them for schools and offices so you can work with natural light without needing additional artificial light. They give great insight in the difference of increasing a window 200mm or moving a wall slightly makes. The below example shows a room with only 1.9% which has multiple but small windows that needs review and will be dark compared to the others which are well lit naturally. With the amount you're going to invest in a basement this could be money very well spent.

 

 

Have you a section? Could you look at stepping the light basement lightwell so at the base it might be only 1.2m but it steps up in raised planters to somehow merge and incorporate into the garden. It will also make it a lot less depressing to look out of a bedroom onto a massive retaining wall.

 

EDIT:

I also think the daylight analysis would be good as I've concerns about you're kitchen.

Edited June 1, 2021 by Dudda

[Strak]

2 hours ago, George said:

It's not a direct answer to your question, but a new build basement under an existing building is not a good idea. Difficult, risky and expensive. The only places that do it are where the land values are super high (i.e. central London) and even then, they can go very badly wrong. Basement conversion is OK but still face challenges with waterproofing. 

 

You'd be better off doing the loft conversion and using the money saved (which would be a LOT) to put in solar panels and an air conditioning system. 

 Thanks for the reply @George - this is actually not under an existing building, it is under part of what is currently the garden which will have a ground floor extension build on top of it. But I know that some of the risks are still there so it could be that we end up with the loft conversion in the end. The main issue is that we can't really get the space that we want from that option.

[Strak]

2 hours ago, markc said:

Looks like you are confusing depth with area, or are you referring to depth on plan? as opposed to vertical depth?

A larger area light well will allow more light in for more hours. Light wells are complex to get right, you need to look at orientation, shadow from other buildings, trees, fences etc.

@markcYes, I realised that the way I wrote it was a bit confusing this morning! I meant the depth on the plan rather than the vertical depth, I just wasn't sure how to say it ?

@Dudda - the daylight analysis sounds like a good suggestion - I will look into it as we're also a bit worried about the kitchen. it's actually one of the things we're hoping to improve on from the current house layout, as the existing room isn't super dark, but definitely is not light. We're hoping that taking out one of the internal walls will help quite a bit with that but hopefully the light survey will confirm that. I quite like the stepping idea too, I'll have a think about that.

[Strak]

@Dudda @markc here's a side elevation of the property which shows the new living area with both basement and loft conversion options: 

side elevation.pdf

[Dudda]

I'd try and keep the basement wall out a bit from the existing house. It will work out easier to build (and therefore cheaper) not having to underpin the existing house as much. My crude overlay also shows how more light can get into the basement if you've the wall stepped and it will look a hell of a lot better than a 2.7/2.8m high retaining wall.

 

The loft will definitely work out cheaper. Basements are expensive and you've a lot of earth to carefully dig out and have only a 2m existing access to the side of the house. I think you need to speak to a structural engineer and builder about this to get a grasp of the costs involved. I'd definitely do that before getting an environmental engineer or wasting time perfecting the layout or size and shape of a lightwell.

[markc]

I would definitely get some specialist input regarding the amount of sunlight / shadow etc.

The rear wall will be a feat in itself and not cheap. Underpinning is pretty straight forward but underpinning and forming the basement wall in one requires a lot of engineering

[Strak]

@markc @Dudda thanks. I actually also asked a basement specialist to look at the plans in detail to give an estimation. They came back with the following proposed approach, estimating around 125k (which as you said, is a lot of money!?

 

In the absence of any structural drawings and on the assumption that the anticipated soils (as above) are realised I have assumed underpinning the rear of the existing dwelling and a ring beam and underpin approach as the most cost effective form of construction as it combines basement retaining walls and temporary stabilisation works into a single construction operation.  For the avoidance of doubt no sheet piles or pile bracing systems are required with this form of construction.

 

Pricing assumes a reinforced concrete sub-structure with an internal cavity drainage system and we will design the structural waterproofing (covered under our PI) which we will guarantee, will comply with current NHBC and LABC requirements for a dual waterproofing system and will deliver a watertight specification to a BS8102 Grade 3 “Dry” environment suitable for habitable space.

 

The attached budgets include structural design, structural waterproofing design, excavation and disposal, temporary works de-watering, the RC basement build, temporary works and associated propping (per design assumptions), structural waterproofing and temporary access requirements only.  Full details of inclusions and exclusions are shown in the attached breakdown Revision 0 however we have assumed that you/others will undertake architectural design, basement internal steel work and lintels, basement internal walls, ground floor, under-build above our RC retaining walls, staircases, insulation, all fit out and all drainage.