Every company on the UK register describes itself with a code. Most people selling to businesses have never used those codes, which is a shame, because they are the closest thing there is to a free, national, self-declared list of who does what.
They are also imperfect in specific and predictable ways, and knowing where they mislead is most of the skill.
What a SIC code actually is
A Standard Industrial Classification code describes a business activity. UK companies pick theirs when they incorporate and confirm it on every confirmation statement. There are more than 600 codes across 21 industry sections, and a company can register up to four of them, with at least one required.
That structure matters when you search. The first two digits are the division, the next digits narrow it. So 41 is construction of buildings, 41201 is construction of commercial buildings, and 41202 is construction of domestic buildings. Searching at division level gives you everyone in the neighbourhood. Searching at five digits gives you a specific trade and misses the companies that classified themselves one notch differently.
Why the codes are unreliable, and in which direction
SIC codes are self-declared. They can be inaccurate or out of date, so check them against other information about the company.
A business can change its activity without updating its codes promptly. Broad codes such as 70229 and 82990 may be useful for a first search but tell you little about the exact services offered. Check the company's own description before contacting it.
There is a change here worth knowing. Under the Economic Crime and Corporate Transparency Act, Companies House can now challenge, reject or remove information that looks incorrect or misleading, and dormant or non-trading codes on actively trading companies are getting more scrutiny. The data should improve. It has not improved yet.
Use codes to build a longlist, never a shortlist
Use a SIC search to build an initial list, then check which companies genuinely fit your services. The code alone cannot establish demand.
Combine the code with things that are harder to get wrong. Incorporation date, because a company formed three months ago has different needs from one formed in 1998. Registered address and its postcode district, which gives you a real radius. Accounts category, which is a rough proxy for size. Officer count. Whether the company has ever filed a charge.
Two or three of those together produce a list you can actually work, and each filter is a fact rather than a self-description.
Find the codes your existing customers use
This is the step that turns a theoretical exercise into a targeted one, and it takes an afternoon once.
Take your last thirty customers. Look each one up on the register and write down their SIC codes. You will find two things. A handful of codes will repeat far more than you expected, and those are your market described in the register's own language. And several customers will carry a code you would never have guessed, which is where the searches you have not been running are hiding.
Do the same with the customers you lost money on, or served badly. Those codes are worth excluding, and an exclusion list is as useful as a target list.
Check recent activity as well as the code
A list of companies in the right sector is not a list of companies who need something today. Code plus event is what makes it useful.
A new incorporation may be worth researching, although it may not yet trade. A charge records secured borrowing, while account and officer changes provide other reasons to investigate. Check the purpose of the event before treating it as a buying signal.
Each of those is on the register, dated, and attached to a company you can already tell is in your market. That combination is the whole method.
The businesses SIC codes cannot see
A whole layer of the economy is missing from this method, and it is better to know that at the start than to conclude your market is small.
Sole traders and ordinary partnerships are not on the Companies House register at all. They have no company number, no SIC code and no filing history, which means an entire category of buyer is invisible to a register-led search. In trades where sole traders are common, a SIC search can quietly exclude most of the market.
Charities are on a different register with a different vocabulary. A registered charity that is also a company carries SIC codes, often a catch-all one, while its actual activity is described in Charity Commission classifications that do not map to SIC. If charities are a market for you, search the charity register directly and treat the SIC code as incidental.
Then there are the branches of overseas companies, limited partnerships and the various establishment forms, each of which files differently. None of this makes the method useless. It sets the boundary of what a longlist built this way can contain, which is the sort of thing worth knowing before you conclude that a campaign underperformed.
Use procurement codes for contract searches
If you sell to the public sector, SIC is the wrong vocabulary and using it will cost you notices.
Public procurement classifies what is being bought using the Common Procurement Vocabulary, which describes the subject of a contract rather than the nature of a company. A CPV code runs to eight digits with a check digit, and it narrows the same way SIC does, from a broad division to a specific item. Every tender and award notice carries at least one.
So a firm has two classifications and they answer different questions. Your SIC code says what you are. The CPV codes on notices say what buyers are purchasing. Find the four or five CPV codes that describe what you sell, set your saved searches on those, and you will stop missing notices written by authors who described the work in words you would never have chosen.
Work backwards to find them. Take three contracts in your category that were awarded to somebody else, open the notices, and read off the CPV codes the buyers used. That is more reliable than picking codes from the list yourself, because it tells you what buyers do rather than what the classification intends.
Getting the whole register at once
Searching one company at a time is fine for research and hopeless for building a list. There is a free bulk option almost nobody outside data teams uses.
Companies House publishes a free company data snapshot every month, as CSV, updated within five working days of the previous month end. It covers active companies and carries the company number, name, registered address, incorporation date, accounts and confirmation statement dates, company status and up to four SIC codes. It is a large download and it opens in anything that reads a spreadsheet, provided you filter before you try to view it.
With that file you can build a targeted list in an evening. Filter to your SIC codes, then to your postcode districts, then to companies incorporated within a date range, and you have a list that would cost real money from a data vendor. There is also a free API if you would rather query than download, and it returns the same underlying record.
The snapshot has one limitation that decides how you use it. It is a picture of the register as it stood, so it tells you who exists rather than what changed. Comparing this month's file against last month's is how you turn it into events, and doing that reliably is a small piece of engineering rather than a spreadsheet task.
A code change is itself a signal
Since a SIC code is confirmed annually, a company that changes one has taken a deliberate step, and the register records the date.
A newly added construction or wholesale code describes a stated activity. It may reflect a change in the business, a correction or an overdue update. Confirm current operations before assuming a new service has launched.
A code change is worth reading alongside the company's other filings. The change records what was declared; it does not establish when the underlying activity began.
A worked example
Say you supply extraction and ventilation systems to commercial kitchens.
The obvious code is 56101, licensed restaurants. It is also the crowded one, and everyone selling to restaurants is looking at it. The useful additions are 56102 and 56103 for unlicensed restaurants and takeaways, 55100 for hotels, 56290 for other food services, which is where contract caterers sit, and 47110 for supermarkets with in-store kitchens.
Now add recent events. An incorporation or food business record can identify a company to research. A low hygiene rating calls for reading the inspection context; it does not establish that ventilation work is needed.
Use the codes to find possible customers and the events to frame your research. Confirm that the business and its requirements fit before making an approach.
Where to look them up
The full condensed list is published by Companies House, and the register itself is free to search and free to read. You can look up any company, see its codes, its filing history and its officers, without an account and without paying anybody.
That is worth saying plainly, because a good deal of what data companies sell is this information with a search box on top. What they add is scale, joining and timing. What they cannot add is accuracy the register never had.