What is a basement conversion?
A basement conversion turns an existing cellar into living space, or excavates a new basement under the house or garden. Waterproofing, structural support and daylight are the main design challenges.
How much does a basement conversion cost in Northumberland?
In Northumberland, a basement conversion typically costs £1,700–£2,900 per m² for converting an existing cellar, including VAT at 20% (standard rate). For a typical 30 m² project that is roughly £51,000–£87,000. The guide for Northumberland reflects typical costs in the North East, roughly 10% below the UK-wide figure. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (21 m²) | £35,500–£61,000 |
| Typical project (30 m²) | £51,000–£87,000 |
| Larger project (42 m²) | £71,500–£122,000 |
| Rate per m² | £1,700–£2,900 |
The biggest cost drivers are:
- Existing cellar or new excavation
- Waterproofing system
- Party wall surveyor fees
- Floor lowering and underpinning
Use the guide price tool to see how size and specification change the figure, then ask for a written, itemised quote.
Do you need planning permission for a basement conversion in Northumberland?
The approval route for Northumberland is set by planning law in England. In England converting an existing cellar with no external change usually does not need planning permission. New excavation, lightwells and changes to the front often do, and some councils, particularly in London, have specific basement policies.
Building regulations approval comes from the council’s building control service or a registered building control approver. The work is inspected at key stages, such as foundations, drainage and insulation, and a completion certificate is issued at the end.
At the survey we check your house’s planning history and any restrictions, and tell you which route applies.
What we handle
Handled as part of most basement conversions:
- Waterproofing design (tanking or cavity drain membrane)
- Lightwells and windows
- Insulation, ventilation and drainage pumps
- Party wall notices
- Electrics, heating and plastering
- Underpinning or new structure where needed
How long does it take?
Build time is typically 8 to 16 weeks for a cellar conversion; longer for new excavation. Allow time beforehand for drawings and approvals, usually 3 to 5 months.
Building in Northumberland
The North East has many terraced streets and post-war semis, where rear extensions and loft conversions are popular ways to gain space.
Towns we cover in Northumberland include Blyth, Cramlington, Ashington and Bedlington.
How it works
- 01
Survey and brief
We visit, discuss your ideas and budget, and survey the property or site at no cost.
- 02
Design and pricing
Design options, then a fixed quote based on the chosen drawings and specification.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about basement conversions in Northumberland
How much does a basement conversion cost in Northumberland?
In Northumberland, a basement conversion typically costs £1,700–£2,900 per m² for converting an existing cellar, including VAT at 20% (standard rate). A typical 30 m² project comes to roughly £51,000–£87,000. Your fixed price depends on the survey and drawings.
Who approves a basement conversion in Northumberland?
Planning matters in Northumberland go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.
Do I need party wall agreements?
In England and Wales, excavating near a neighbour’s foundations usually triggers the Party Wall etc. Act 1996.
How do you keep a basement dry?
With a waterproofing system designed to BS 8102, typically a cavity drain membrane with a sump pump, or a tanking system.
Is new excavation worth it?
It can be in high-value areas, but costs are high. We give a feasibility view before design starts.