Case Study · Basement Conversion
From dark shell to daylight basement — a lower-ground conversion in Llangollen
How we turned a cold, below-ground shell built into the hillside into a bright, fully-finished daylight basement — a spacious lounge, a practical utility and a smart shower room, with patio doors straight out to the garden.
The brief
Sion’s home in Llangollen is built into the side of a hill — so while the front looks like a two-storey house, the back is actually three storeys, with a full lower-ground floor tucked beneath the living space. That floor had never been used for anything and was completely closed off: a bare, empty shell with cold walls, no heating and no proper access.
Sion and his family had always wanted to turn it into a real part of the house — a comfortable living space they could actually use every day. The wish list was a proper lounge, a separate utility to take the laundry and clutter out of the main house, and a shower room downstairs. On paper it felt like a huge, daunting job. Our role was to take the whole thing on and manage it from bare shell to finished, decorated rooms.
What is a daylight (walk-out) basement?
Because the house is built into a slope, this wasn’t a basement in the “dark box underground” sense. It’s what’s known as a daylight basement — sometimes called a walk-out basement.
Daylight basement, in plain English
On a sloping plot, the ground is high at one side of the house and low at the other. That means part of the lower floor is buried against the hillside (the rear, here), while the opposite side (the front) sits at garden level — so you can put in full-height windows and doors and literally walk out into the garden.
The payoff is a lower-ground room that gets real natural light and proper access, and feels like a normal room rather than a cellar. The challenge is that the buried side has to be dealt with properly for damp and cold before any of that finish goes on — which is where most of the skill in a job like this lives.
What we did
We took the space from bare shell to finished rooms, managing every trade ourselves so Sion never had to chase plumbers, sparks or plasterers around. The work fell into a few key stages.
Access — a new staircase
First we formed a new opening in the floor above and installed a staircase down from the main house, so the lower-ground floor finally connected properly to the rest of the home instead of being a separate, awkward space.
Beating the damp — tanking the buried wall
The rear wall sits below ground against the hillside, so before anything else it had to be waterproofed. We tanked the buried side to hold back moisture, then battened out and lined every wall with insulated plasterboard, and insulated the ceiling too. That’s the difference between a cold, damp cellar and a warm, dry room you’d happily sit in on a winter evening.
Letting the light in — patio doors to the garden
On the front, garden-level side we fitted patio doors, turning the darkest corner of the house into its brightest — and giving the family a direct walk-out to the garden. This is exactly what makes a daylight basement work.
The clever bit — drainage with a macerator
The trickiest part of any below-ground bathroom is waste. Because the new room sat lower than the existing drains, you can’t simply let the toilet, shower and sink drain away by gravity — there’s nowhere for it to fall to. The answer was a Saniflo macerator: it collects the waste from the toilet, shower, basin and the utility’s sink and washing machine, then pumps it up and away into the home’s existing soil stack. It’s the detail that makes a downstairs shower room possible at all in a spot like this.
First fix, finish and decoration
From there we ran all the first-fix plumbing and electrics — new sockets, spotlights, an extractor fan, heating moved to suit the new layout — then plaster-boarded, plastered, fitted the shower room and utility, and finished with skirting, architrave and paint to match the rest of the house.
The build, step by step
Here's how the conversion came together, in the order it actually happened — from the bare, closed-off shell to a finished room the family live in every day.
The starting point
We started with a cold, closed-off shell. The walls were bare blockwork, the floor was raw concrete and the only light came from a single strip light. Before any of the nice stuff, this was what we had to work with — so the first jobs were protecting the space and setting out exactly where everything would go.
Getting materials to a hillside site
Because the house sits on a steep hillside, even getting materials in took planning. Insulated plasterboard, timber and fittings arrived by crane-lorry out on the road, then had to be brought down to the lower-ground floor by hand — a reminder that a sloping site adds work before a single wall goes up.
Propping and forming the stair opening
To connect the floor to the rest of the house we needed a new staircase, which meant forming an opening in the floor above. We supported the joists with acrow props and framed the opening safely before cutting through, so the structure overhead stayed fully carried at every stage.
Building the new staircase
With the opening formed, we built and set the new staircase into place. Getting the rise and going right on a basement stair is fiddly — it has to land neatly at both the top and the bottom — so this stage takes patience to get spot on.
Boxing in the stairwell
We then boxed in and plasterboarded around the flight and its underside, tidying the staircase into the room and turning what could have been an awkward gap into a useful understairs space.
Insulating the ceiling & running the waste
Overhead, we insulated between the joists to keep the new floor warm, and ran the waste pipework up to the macerator so the shower, basin and toilet could drain away. This below-ground plumbing is exactly what makes a downstairs bathroom possible in a room that sits lower than the drains.
Tanking and insulated boarding
Once the buried wall was tanked against damp, we battened out and lined the walls throughout with insulated plasterboard. This is the step that turns a cold, damp shell into a warm, dry room — and it's why the finished space feels like any other in the house rather than a cellar.
Forming the shower room
We studded out and first-fixed the new shower room — framing the walls, setting the tray and running the hot, cold and waste — ready for boarding and tiling. A neat, private shower room downstairs was right at the top of Sion's wish list.
Plastering — and the patio doors go in
With the patio doors fitted on the garden side, the whole floor was skimmed and plastered. Even at this bare-plaster stage you can see how much daylight the doors pull in — the room already reads as a proper living space rather than a basement.
Decoration and finishing
Next came the finishing: walls painted, skirting and architrave fitted to match the rest of the house, and the staircase finished in white with a full balustrade. At this point everything was done bar the carpet.
The finished space
And here's the payoff — a bright, warm lower-ground floor the family use every day, with the garden just through the patio doors. From a closed-off shell to one of the favourite rooms in the house.
The result
Where there was once a cold, unused shell, Sion’s family now have a whole extra floor of proper living space: a bright lounge with the garden just through the patio doors, a hard-working utility that keeps the laundry out of the main house, and a smart shower room — all warm, dry, well-lit and finished to match the rest of the home. It genuinely added a storey’s worth of usable space to the house without changing its footprint.
Basement conversions — your questions answered
What is a daylight or walk-out basement conversion?
It’s converting a lower-ground floor on a sloping plot into finished living space. Because the ground is lower on one side, that side can have full windows or patio doors and a walk-out to the garden, so the room gets real daylight and proper access instead of feeling like a cellar. Sion’s home in Llangollen is a textbook example — two storeys at the front, three at the back.
Do you need planning permission to convert a basement?
Converting an existing below-ground space into a habitable room is usually a change of use rather than a new build, so it often doesn’t need full planning permission — but it does need to meet Building Regulations for things like damp-proofing, ventilation, fire escape and electrics. If you’re adding or enlarging windows or excavating, that can change things. We’ll always tell you honestly what a specific job needs before we start.
How do you stop a below-ground room being damp?
Any wall that sits against the ground has to be waterproofed before it’s finished. On Sion’s conversion we tanked the buried rear wall to hold back moisture, then battened out and lined the walls with insulated plasterboard and insulated the ceiling. Done properly, the finished room stays warm and dry all year — the damp management is the single most important part of the job.
How does a toilet or shower work if it’s below the drains?
When the new room sits lower than the existing drains, waste can’t drain away by gravity. The solution is a macerator pump — a Saniflo unit — which collects the waste from the toilet, shower, basin and any utility appliances and pumps it up into the home’s existing soil stack. It’s what makes a downstairs shower room possible in a spot where a normal connection simply couldn’t work.
Will a converted basement be warm enough to live in?
Yes — if it’s insulated properly. We line the walls with insulated plasterboard, insulate the ceiling, and set up heating to suit the new layout. That turns a cold shell into a room that’s as comfortable as any other in the house.
Can you get natural light into a basement?
On a sloping plot, absolutely. Because one side of the floor is at garden level, we can fit large windows or patio doors there — as we did for Sion, where the patio doors turned the darkest part of the house into its brightest and opened straight onto the garden.
How long does a basement conversion take?
Sion’s full conversion — staircase, tanking, insulation, a shower room, a utility, patio doors and full decoration — took around six weeks. Timescales vary with size and spec, but we’ll give you a realistic programme with your quote and manage every trade so it runs to plan.
Do you carry out basement conversions across North Wales?
Yes. We’re based near Wrexham and cover Llangollen, Denbighshire and the wider North Wales and Chester area for basement and lower-ground conversions, extensions, loft and garage conversions and full renovations.
“We’ve always wanted to have the floor of our house converted into a living space. It felt like a massive job to us. Andy absolutely nailed every aspect of the build. It was a bare empty shell which needed electrics, plumbing, staircase, plastering and all the other bits in-between. We had the area converted into a lounge, utility room and shower room. Andy’s advice on what could be done, where to place things like sockets, sinks etc was invaluable. Andy project managed the whole build so we didn’t need to speak with various trades about timings and organising. I would highly recommend Castlegate Building and Maintenance for any job!”— Sion, Llangollen · Read the full review on Google →
Thinking about converting your basement or lower-ground floor?
If you’ve got an unused cellar, lower-ground floor or space built into a slope, we’ll take a look and give you a straight answer and a clear, no-obligation quote.
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