Starting a business in the UK is an exciting but daunting journey. The first 90 days are critical for setting up your company’s financial and operational systems correctly. Many founders stumble early on due to missed registrations or poor record-keeping, which can lead to fines or cash flow issues. This comprehensive week-by-week checklist will guide you from day one, ensuring you hit key milestones like HMRC registration, issuing your first invoice, and reviewing your expenses systematically. Whether you’re a sole trader, partnership, or limited company, these practical steps will help you build a strong foundation for your startup.
Week 1: Register with HMRC and Understand Your Tax Responsibilities
Your very first priority as a UK founder is to register your business with HM Revenue & Customs (HMRC). This step is crucial because it ensures you comply with tax laws and avoid penalties. Different business structures have different registration requirements. Sole traders must register as self-employed, while limited companies need to register for Corporation Tax with Companies House and HMRC.
For sole traders, registration must be done within three months of starting trading to avoid a penalty. You can register online through the HMRC website, and you will receive a Unique Taxpayer Reference (UTR). As a limited company, registration happens automatically when you incorporate via Companies House, but you still need to notify HMRC separately to set up your Corporation Tax account.
See alsoStripe Payment Links: the easiest way to get paid in 2026→- Register as self-employed if you’re a sole trader: https://www.gov.uk/set-up-sole-trader
- Incorporate your company via Companies House: https://www.gov.uk/limited-company-formation
- Register for Corporation Tax within three months of starting business activities: https://www.gov.uk/corporation-tax/register
- Set up a business bank account early to keep personal and business finances separate.
Missing your HMRC registration deadline can lead to hefty fines and complicate your tax filings. Registering within the first week ensures you receive your UTR and can submit Self Assessment or Corporation Tax returns on time.
Understanding your tax responsibilities early will save you headaches later. Keep in mind the UK tax year runs from 6 April to 5 April, so your first tax return may cover a partial year depending on when you start. Also, consider whether you need to register for VAT. If you expect your taxable turnover to exceed £85,000 in any 12-month period, VAT registration is mandatory.
Weeks 2-3: Set Up Your Accounting and Record-Keeping Systems
With HMRC registration underway, it’s time to set up your bookkeeping and accounting systems. Accurate record-keeping is a legal requirement and essential for managing your cash flow and preparing financial statements. Many UK startups use cloud accounting software like Xero, QuickBooks, or FreeAgent, which are tailored for UK tax rules and integrate directly with HMRC’s Making Tax Digital (MTD) requirements.
Ensure you understand what records you need to keep: invoices, receipts, bank statements, and mileage logs if you use a vehicle for business. Digital copies are acceptable, but they must be accurate and complete. If you’re not confident handling bookkeeping yourself, consider hiring a part-time accountant or bookkeeper familiar with UK startups.
- Choose cloud accounting software compliant with Making Tax Digital.
- Create a folder system (digital or physical) for invoices, receipts, and contracts.
- Set up a separate business bank account with a UK bank for transparency.
- Keep track of all business expenses, including small purchases.
Cloud accounting tools can automatically import bank transactions and categorise expenses, saving you hours each week. Setting this up in your first three weeks helps avoid a backlog and reduces errors come tax time.
Week 4: Send Out Your First Invoice
By the end of your first month, you should be ready to invoice your first customer or client. Sending your first invoice is a major milestone and sets the tone for your cash flow. UK invoicing must include specific information to comply with HMRC rules and ensure timely payment.
Your invoice should clearly state your business name, address, and contact details, plus the customer’s details. Include a unique invoice number, invoice date, and payment terms (e.g., 30 days). If you’re VAT registered, you must show your VAT registration number and the VAT breakdown on the invoice.
- Business name and address
- Customer name and address
- Unique invoice number
- Invoice date
- Description of goods or services
- Total amount payable
- VAT amount and registration number (if applicable)
- Payment terms and methods accepted
Using an invoicing template within your accounting software ensures you don’t miss any details. Send invoices promptly after delivering your product or service to speed up payment. Follow up politely on late payments, as cash flow is often the biggest challenge in early trading.
Stipulate your payment terms clearly on every invoice and consider offering early payment discounts or charging interest on overdue payments within legal limits. Good invoicing habits help maintain healthy cash flow.
Weeks 5-11: Build Operational Habits and Prepare for Your First Expense Review
During weeks 5 to 11, focus on developing consistent operational habits. This includes logging all income and expenses, reconciling your bank statements regularly, and keeping communication open with suppliers and clients. Staying organised prevents last-minute scrambles come tax season.
Use this period to also track your mileage and other deductible expenses, like home office costs or software subscriptions. Understanding what counts as an allowable expense helps reduce your taxable profit and saves you money. Not all expenses are straightforward, so keep detailed notes and receipts.
If you’re a limited company, consider your salary and dividends strategy now. Paying yourself a combination of salary and dividends can be tax-efficient but requires careful planning. Consult an accountant if unsure.
Week 12: Conduct Your First Proper Expense and Financial Review
At the end of your first 90 days, it’s time to review your finances thoroughly. This review helps you understand your cash flow, profits, and areas where you can cut costs. It’s also a great time to prepare for upcoming VAT returns if you’re registered, or to get ready for your Corporation Tax or Self Assessment return.
Start by reconciling your bank statements with your accounting records to spot any missing transactions. Check that every expense is categorised correctly and supported by receipts or invoices. Review your income streams and identify any late payers or potential bad debts.
- Reconcile bank statements with your accounting software
- Verify all expenses have receipts and are allowable
- Analyse your cash flow trends and forecast next quarter
- Prepare VAT return drafts if applicable
- Set reminders for upcoming tax deadlines (Self Assessment, Corporation Tax)
This is also the right moment to reflect on your pricing strategy and client payment terms. If you notice recurring issues like late payments or high overheads, plan corrective actions. Solid financial discipline in these early months will pay dividends as your business grows.
“Reviewing my expenses and cash flow after three months was a game-changer. It highlighted unnecessary subscriptions I cancelled and helped me renegotiate payment terms with clients. That early financial clarity gave me confidence to scale.” – Sarah J., London-based digital marketing startup founder
Beyond Day 90: Next Steps and Ongoing Compliance
Completing your first 90 days doesn’t mean your work is done. Business finances require ongoing attention. You must continue submitting quarterly VAT returns if registered, file annual accounts and tax returns, and keep up with changes in tax legislation. Consider setting quarterly reviews to stay on top of your financial health.
As your business grows, you may want to explore additional registrations such as PAYE for employees or sector-specific licences. Keep your business insurance up to date and review your bookkeeping processes regularly to adapt to increasing complexity.
Finally, don’t hesitate to seek professional advice. Accountants, business mentors, and industry peers can offer invaluable insights tailored to your UK startup’s specific needs.