Riordan O’Sullivan & Co has been providing expert accounting and tax solutions to businesses and individuals throughout London and the South East of England for over 50 years.

Riordans have been advising businesses and private clients for generations.

Founded in 1968, we have evolved through the decades to become one of London’s most established accountancy firms. We now offer a wide suite of accounting, tax planning and advisory services to businesses and private clients in search of practical solutions from an experienced team with a refreshingly proactive and hands-on approach.

Our client base extends into a wealth of industries.

We work with an impressive portfolio of clients in a variety of sectors, from the beginner embarking on their first dream to businesses with turnovers ranging from thousands to hundreds of millions.

Many of these firms came to our practice in the 1960s, 1970s and 1980s and have stayed with us ever since – and these businesses are now well into second generation of owners, something that we believe is testament to the quality of the service we offer and the trust that our clients have placed on us.

In particular, we never forget that our largest clients were beginners themselves one day.

Our reputation is second to none

Our clients regard us not only as their accountants, but as an extension of their own business and personal financial affairs. They value our practical advice, our efforts to find a solution, our flexible and approachable nature; they appreciate our enduring transparency; and they enjoy the reassurance that comes with partnering with one of London’s long established and most trusted accountancy firms.

We do things differently

We offer so much more than a cut and dry accounting approach. Regardless of the challenges you are facing or the complexity of your financial situation, we will take the time to immerse ourselves in your circumstances to gain a thorough understanding of your requirements - we will stand in your shoes. From there, we will provide you with the knowledge and the tools you need to achieve the best possible outcome for yourself, your family and your business.

We have specialist experience in the construction property and waste management space

We boast a dedicated and experienced team that services the specialist requirements of businesses in the construction, property and waste management sector.

These highly experienced professionals work to deliver jargon-free accounting, tax planning, financial management, costing, restructuring and compliance solutions to these firms.

We also offer advice on the laws and regulations around outsourcing of trades and professionals for these sectors. For example, we can help with IR35, contracts and status determinations.



Stay informed with our downloadable essential factsheets from HMRC and Companies House on our Factsheet page.

Download our fact sheets


Call us now on 020 8969 9411 or leave your details here and we’ll get back to you.