TBCs in specifications at tender
Why open TBCs at tender become variations at contract, how to run a TBC register with owners and dates, and how to close, convert or allocate every item before issue.
Why is a TBC at tender a liability?
A TBC left in a specification at tender is a liability because the contractor prices around it, and what is open at tender is usually still open at contract, so the decision arrives later as an instruction with cost and time attached. The fix is to manage TBCs as a register with owners and dates, and to close, convert or explicitly allocate every one before issue.
TBC stands for "to be confirmed". It marks a value, product or decision that has not yet been made. TBCs at Stage 4 are normal: technical design is where decisions are still being made, and marking them clearly is good practice. The problem is TBCs that survive into documents the contractor prices and signs.
Avoice's State of UK Specifications 2026 graded 50 real UK specifications and tracked open TBCs by stage.
The trend is downward, so practices are closing TBCs. Even so, three quarters of the documents that became contract documents still contained explicit unresolved items.
Four ways an open TBC costs you
An open TBC is not a neutral placeholder. It is priced badly, resolved by instruction, blurs design responsibility and weakens the design record.
- 01
Not priced properly
Tenderers price the cheapest option, the dearest, or exclude it. Competition pushes to the cheapest.
Tender price - 02
Becomes an instruction
Under the SQI rubric an open TBC on a priced item is a major defect: it takes an instruction to resolve.
Variation · programme - 03
Shifts design risk
"Acoustic rating TBC" leaves unclear whether the designer or the contractor owes the decision.
Design responsibility - 04
Weakens the record
Open fire, structural or thermal items at contract are hard to present as a well-managed design.
Part 2A · golden thread
Because competitive tendering favours the cheapest plausible reading, the decision made later almost always costs more than the tender allowed.
Instructions after contract are variations, and variations bring cost, programme and dispute risk. Unclear design responsibility is a common source of disputes in its own right.
The record point matters more since 1 October 2023. Designers in England have duties under Part 2A of the Building Regulations 2010, inserted by SI 2023/911, to take all reasonable steps to plan, manage and monitor design work so that it complies. On higher-risk buildings, the specification also feeds the golden thread.
What is a TBC register?
A TBC register is a single list, kept outside the specification, of every open item across the project's specification and drawings. Each line says what is open, why, who owns it and what happens if it is not closed in time.
A bare TBC
Acoustic rating: TBC
Tells nobody anything. No one knows who to chase or when it is needed.
A register line
Acoustic rating TBC because the acoustician's report has not yet been received.
Tells the project lead exactly who to chase.
Most TBCs are waiting on someone else: a client choice, a consultant report or a manufacturer's data. Recording why makes the chase obvious.
Fields a useful register needs
- Reference
- The clause or drawing reference where the TBC sits.
- Item
- What is unresolved, in one line.
- Reason
- Why it is open: missing information, client decision, consultant input or product selection.
- Owner
- The person or organisation who must resolve it: the client, a consultant, a manufacturer or the design team. The specification author records it; ownership sits with whoever can close it.
- Needed by
- The date or issue by which it must be closed.
- Priced?
- Whether the item affects the tender price.
- Fallback
- What happens if it is not closed in time: provisional sum, contractor design or hold.
- Status
- Open, closed, converted or allocated.
How to burn TBCs down before tender issue
Treat TBC closure as a planned activity with a target of zero priced TBCs at tender, not a tidy-up the night before issue.
- Capture every TBC as it is created. Whoever writes "TBC" into a clause adds the register line at the same time, with a reason and owner.
- Review the register at every design team meeting. Sort by "needed by" and chase owners.
- Set a cut-off two to three weeks before issue. Every remaining TBC must go down one of four routes.
- Sweep the documents before issue for every placeholder form your team uses.
- Re-check at contract. Tender queries and post-tender changes create new TBCs, so run the sweep again before the contract documents are finalised.
A provisional sum differs from a TBC because every tenderer prices the same stated figure, and the final cost is adjusted through the contract. A bare TBC has no price basis, so each tenderer handles it differently.
The pre-issue sweep
- Every specification and drawing searched for TBC, TBA and TBD.
- Searched for "???" and "[insert]" placeholders.
- Highlighted text and empty fields checked.
- Anything found that is not in the register added and resolved; it is a process failure.
- Sweep repeated before the contract documents are finalised.
Which TBCs should be closed first?
Prioritise by consequence, not by convenience. Performance values that could become critical defects go first; finishes that can sensibly sit as provisional sums go last.
- Fire, structural and thermal performance values. These can become critical defects if resolved the wrong way.
- Items that change the price significantly: facade systems, plant, lifts and specialist joinery.
- Items affecting other trades: anything with interfaces, such as service penetrations through fire compartments.
- Finishes and fittings that can sensibly sit as provisional sums.
Close one TBC, check every mention
TBCs are one of six defect classes in the SQI, alongside internal contradictions, orphan references, leftover master content, drawing-spec mismatches and withdrawn standards. Closing a TBC often creates a new risk: the decision is made, one clause is updated, and another clause or drawing that mentioned the same element is not. Follow every closure with a check of every other mention of that element. The most common specification defects at contract issue sets out the full checklist.
A TBC at Stage 4 is good practice. The same TBC at contract is a variation waiting to happen.
Where Avoice fits
When Avoice spec writing cannot fill a value from the information supplied, it does not guess. It emits an explicit TBC with the reason, and those TBCs roll up into a per-project TBC register.
For example, an acoustic rating is marked to be confirmed because the acoustician's report has not been supplied. The register shows what is open, why and where it sits, so the burn-down can start at Stage 4 instead of the week of tender. Avoice's QA/QC then flags any TBCs still open at issue.
Software can record, track and flag. The design team still makes the decisions that close each item.
See the TBC register on a live project
Spec writing that marks missing information as a TBC with a reason, rolls every open item into a per-project register and flags anything still open at issue.
This guide is for general education and is not professional or contractual advice. Figures are from Avoice's State of UK Specifications 2026. Agree how open items are handled with the client, cost consultant and contract administrator for your project.