Hiring the wrong contractor for a basement conversion can cost you tens of thousands of pounds and leave you with a space that fails building regulations or can't be legally sold on. Asking the right questions before you commit takes less than an hour and filters out the firms that aren't up to the job. Here is exactly what to ask and what a solid answer looks like.

1. Do you carry public liability insurance — and can I see the certificate?

Any reputable basement conversion specialist should carry public liability insurance of at least £1 million, and most carry £2 million or more. Ask to see the actual certificate, not just a verbal assurance. If a contractor working in a confined space causes structural damage to your property or a neighbouring one, you need to know you are covered. No certificate means no contract.

2. Will you handle the Scottish building warrant application?

In Scotland, a basement conversion almost always requires a building warrant from your local authority before work begins — this is a legal requirement under the Building (Scotland) Act 2003. A good specialist will either manage the application on your behalf or guide you clearly through it. Be wary of anyone who waves the question away or tells you a warrant "probably won't be needed." It usually is, and skipping it creates serious problems when you come to sell.

3. What experience do you have with waterproofing and tanking systems?

Below-ground spaces in Scotland face significant moisture ingress, particularly in older Edinburgh tenements and sandstone properties. Ask the contractor which waterproofing method they use — cavity drain membrane systems, cementitious tanking, or a combination — and whether they hold certification from a recognised body such as the British Standard BS 8102 compliance framework. A good answer will include references to the specific ground conditions in your area, not a one-size-fits-all approach.

Most important question of all: Ask for the contact details of two or three recent clients whose basement conversions you can actually visit. A contractor confident in their work will give you names without hesitation. One who hedges is telling you something important.

4. Who carries out the electrical work, and are they registered with SELECT or NICEIC?

Electrical installation in a basement must be carried out by a qualified electrician. In Scotland, the relevant body is SELECT (the Electrical Contractors' Association of Scotland). Ask whether the electrician working on your project is SELECT or NICEIC registered, and confirm that all electrical work will be certified on completion. Damp environments and electricity are a serious combination — this is not a place to cut costs.

5. Do you work with a structural engineer, and who pays for that?

Lowering a floor slab, underpinning foundations, or forming new openings all require structural engineer sign-off. Some contractors include this in their quote; others treat it as an extra. Clarify upfront who commissions the engineer's report, who pays for it, and how any resulting specification changes are handled. If a contractor tells you they don't need an engineer for significant structural work, walk away.

6. What does your quote actually include — and what is excluded?

Basement conversions have a well-earned reputation for scope creep. Get a fully itemised written quote that lists what is and is not included: groundwork, waterproofing, drainage, first and second fix, plastering, decoration, building warrant fees, and VAT. Ask specifically about what happens if unforeseen issues arise — rotted joists, hidden pipework, or poor ground conditions. A reputable firm will have a clear change-order process rather than springing costs on you mid-project.

7. What SVQ or trade qualifications do your site workers hold?

Ask whether the people physically doing the work hold relevant Scottish Vocational Qualifications (SVQs) or equivalent trade qualifications, and whether they carry a valid CSCS card (Construction Skills Certification Scheme). On a complex underground project, experience and training matter considerably more than a polished website. A good contractor will answer this confidently and specifically.

8. What is your process if the work causes damage to a neighbouring property?

In Edinburgh's tightly packed Victorian and Georgian streets, basement excavation can affect shared walls and neighbouring foundations. Check whether the contractor is familiar with Party Wall agreements (which apply in Scotland under common law and specific tenement rules), and confirm how disputes or damage claims would be handled. A specialist who has worked in the city before will know this territory well.

Checklist at a glance

  • Do you carry public liability insurance, and can I see the certificate?
  • Will you handle the Scottish building warrant application?
  • What waterproofing systems do you use, and are you BS 8102 compliant?
  • Is your electrician SELECT or NICEIC registered?
  • Do you work with a structural engineer, and who covers that cost?
  • What exactly is included and excluded in your written quote?
  • What SVQ or trade qualifications do your site workers hold?
  • How do you handle damage to neighbouring properties?
  • Can I visit a recent completed project and speak to the client?

Once you have your answers, you will be in a much stronger position to compare contractors fairly. To get started, find a trusted basement conversion specialist in Edinburgh through the Found Local directory, where all listed businesses are verified and reviewed by local customers.