Construction design package - Your planning permission is already approved and all site investigations and reports — including the full geological report — are completed and you are looking choosing the right basement company. At this stage, our package provides all documentation required for our specialised basement construction methodology: 3D architectural design, structural engineering calculations and drawings, building control documentation, and comprehensive site management plan at the cost of £5,760 + VAT - Flat rate not affected by the basement size or complexity

Do Not include building control fees.

Unlike traditional methods, our basement cost calculation is based on the external shell volume, rather than square metres. This approach allows you to set a realistic budget from the start and play out the potential size of your completed underground steel-shell basement with greater confidence.

The price for excavations and complete steel-shell installation, including concrete encasement, soil backfilling and 600mm of surface soil fill, drainage and levelling, is £1,153 per cubic metre + VAT.

To work out the size of your basement shell, you only need a few quick measurements. The calculation takes less than a minute.

1. Set your finished room height.

For example, if you want a finished ceiling height of 2.4m, add 0.8m for the internal and external structural layers. Finished ceiling height can typically range from 2.1m to 2.7m, your decision.

This gives a basement shell height of 3.2m.

2. Set your finished width and length.

For example, you may want a finished room measuring 4m wide and 10m long. The width can typically range from 3m to 7m, and the length can extend to 30m or more, depending on the site and design.

Add 0.66m to each side to allow for the steel shell and structural layers.

Your shell dimensions become:

4.66m × 10.66m × 3.2m

3. Calculate the volume.

Multiply the three dimensions:

4.66 × 10.66 × 3.2 = 158.96 cubic metres

The final volume — 158.96 m³ — is used to calculate the basement shell cost: 158.96 × £1,153 = £183,283 + VAT.

The under-garden hallway and staircase structure is optional. It is only required if you want the basement to function as a fully integrated part of the house. This connection creates a seamless transition between levels and helps the basement feel like a natural extension of the home.

The alternative is to keep the basement independent, similar to a back-garden summer house. In that case, you can use simple side access, costing only the floating steps, together with a rear courtyard that serves as the second entrance or required emergency escape. Both options are fully compliant; the right choice depends on how you want to use the space.

The cost is calculated at the same rate as the basement shell: £1,153 per cubic metre + VAT.

To work out the size of your basement hallway shell and staircase connection, you only need a few quick measurements — it takes less than a minute.

1. Set the finished hallway height.

For example, if you want a finished ceiling height of 2.4 m, add 0.8 m to allow for the internal and external structural layers.

This gives a basement shell height of 3.2 m.

2. Set the finished width and length.

For example, you may want a finished hallway 1.4 m wide and 5 m long, reaching the house at the point where the staircase begins.

Add 0.8 m to the width only to allow for the steel shell, structural layers, and the utility shaft running along one internal side wall.

The shell dimensions are therefore:

2.2 m wide × 5 m long × 3.2 m high

3. Calculate the shell volume.

Multiply the three shell dimensions:

2.2 × 5 × 3.2 = 35.2 m³

The final volume — 35.2 m³ — is used to calculate the hallway shell cost: 35.2 × £1,153 = £40,585 + VAT.

The staircase shell, measuring 4.5 m × 2.2 m and including up to 19 steps, is charged at a standard price of £24,650 + VAT.

Courtyard way-out floating steps are charged at £220 each: 19 steps × £220 = £4,180 + VAT.

Note:

- Any open side access or courtyard requires an integrated rain waters  drainage system.

- In many properties, the main waste pipe and manhole are located at the rear of the house, which may affect the required hallway depth and shell height.

Integration of drainage systems is optional but essential when you planning to have a toilet, a bathroom, landry room, kitchen or open spaces like side entrance or courtyard. When required, we install an 800-litre twin sewage pumping station, consisting of a 1000 mm × 1000 mm polyethylene moulded chamber, two solids-handling pumps capable of a 10 m pumping head, four float switches, an LR40 automatic dual-pump control panel, a non-return valve, and an active overflow alarm.

The chamber is set in concrete below the basement slab and connected watertight to the steel-shell floor, with an inspection cover for access. From there, the outlet waste pipe is connected directly to the property’s main manhole, providing reliable and compliant waste removal for all basement plumbing.

The complete rain waters (for open spaces like side access or courtyard), bathroom and kitchen drainage system is priced at £12,990 + VAT.

A courtyard is optional, but any open courtyard or side access must include a suitable rainwater drainage system, with drainage pipework installed beneath the basement floor. If you want more natural light and ventilation in the basement, a courtyard becomes a valuable design feature. When the basement is connected to the house by stairs or a lift, the courtyard can also provide the required secondary entrance and emergency escape route for Building Control compliance. It brings daylight, natural ventilation, and safety compliance together in one feature, helping the basement feel brighter, safer, and more connected to the home. The courtyard is priced at the same rate as the basement shell: £1,153 per cubic metre + VAT. The only difference is that the perimeter walls extend 0.6 m above basement level to meet the finished garden floor, so for calculation purposes, it should be treated as part of the basement shell height.

Note: Building Control requirements are strict when providing a safe emergency exit from a basement. If your design does not include a courtyard, the secondary escape route must be provided by a ceiling hatch positioned on the opposite side of the house. A spiral staircase can then be installed through the hatch to create a compliant, compact, and efficient secondary exit. The standard hatch size is 1 m × 1 m, and it requires a specially engineered structural ceiling platform. The perimeter steel walls are extended upward to meet the finished garden floor level, creating a strong structural frame for the insulated, watertight hatch.

Hatch cost is £5,330 + VAT

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A retractable courtyard roof is optional, but it requires specific upgrades to the steel-shell structure to integrate the moving platform. The insulated roof operates on a track system and can be opened or closed at the touch of a button. When fully opened, the mechanism slides away and is housed above the main basement ceiling inside a fully waterproof casing. At ground level, it appears as a clean, solid deck raised approximately 200 mm above the finished garden level.

The purpose of the retractable roof is simple: it keeps rain out of the courtyard, improves security, and provides privacy when required, while still allowing natural light and ventilation when open.

The cost is £1,795 per square metre + VAT. As a practical example, if your courtyard measures 4 m wide by 2.5 m deep, the retractable roof size would be 4.14 m × 2.97 m = 12.3 m². The cost would therefore be 12.3 × £1,795 = £22,078 + VAT.

The visible deck above ground would measure approximately 4.40 m × 3.2 m.

A walk-on triple-glazed skylight is optional, but it is an ideal solution when you want natural light in a specific area of your under-garden basement, such as a children’s playroom or bedroom. The maximum size for a single skylight is 0.83 m wide × 3 m long, and it requires a specially engineered structural ceiling platform. The perimeter steel walls are extended 0.6 m upward to meet the finished garden level, creating a strong structural frame for the skylight.

We use a frameless installation system designed to remain watertight for decades, with anti-slip walk-on glass built to withstand foot traffic, weather exposure, and long-term outdoor use. From above, it delivers a clean, minimal finish; below, it brings bright natural light into the basement.

The cost is £7,180 per square metre + VAT.

Example: a full-size skylight measuring 0.83 m × 3 m = 2.49 m². At £7,180 per square metre, the cost is 2.49 × £7,180 = £17,878 + VAT.