A semi-basement is a half-sunken floor that sits partially below ground level but still receives daylight through windows. The difference from a cellar is that a cellar is completely underground, while a semi-basement partly rises above ground. Semi-basements are common in townhouses and canal houses.
Semi-basement vs. cellar vs. crawl space
| Feature | Semi-basement | Cellar | Crawl space |
|---|---|---|---|
| Position | Half below ground | Fully below ground | Under ground floor |
| Daylight | Yes (windows at street side) | No (or light well) | No |
| Standing height | 2.10+ m | 2.00+ m | 40-80 cm |
| Function | Living, working, hospitality | Storage, utilities | Pipes, access hatch |
| Ventilation | Windows and/or mechanical | Mechanical | Ventilation grilles |
Using as living space
A semi-basement may be used as independent living space if it meets:
- Minimum ceiling height — 2.10 m (building regulations)
- Daylight — Minimum 10% of floor area as glass area
- Ventilation — Mechanical extraction or openable windows
- Emergency exit — Accessible escape window or second exit
Moisture problems
The biggest concern with semi-basements is damp. Because the walls sit partly in the ground:
| Problem | Cause | Solution |
|---|---|---|
| Rising damp | Groundwater wicks up through masonry | Injection (silicone), waterproof coating |
| Penetrating damp | Rainwater pressure against the wall | Install drainage, waterproof render |
| Condensation | Cold surface + warm indoor air | Ventilate, insulate walls |
| Leakage | Cracks in floor/wall | Repair + waterproof layers |
Renovating a semi-basement — DIY
1. Identify moisture source — Rising, penetrating or condensation?
2. Check drainage — Is there drainage around the building?
3. Waterproof coating — Treat interior walls with waterproof mortar
4. Insulate — Internal wall insulation (use moisture-regulating boards, not standard EPS)
5. Improve ventilation — Mechanical extraction or ventilation grilles
6. Floor — Waterproof layer + insulation + screed
Related terms
- Foundation
- Crawl space
- Frost line
- Drainage
- Reinforcement
- Vapour barrier