The deal for the Windsor-based firm excludes the audit practice and comprises £4.25m in cash with the remainder through the issue of new ordinary shares.
It follows the recent £10.25m acquisition of mid-tier firm Morison Stoneham last month.
Tenon chief executive Ian Buckley, said: ‘We see the strengths of WA in outsourcing, financial services and information technology consultancy.
‘While we begin the process of integrating WA and MS we will also continue our discussions with other high quality practices which we believe will make a major contribution to the growth of Tenon.’
Williams Allan is made up of two accountancy partnerships and two companies, and recorded a £5.6m turnover for the last financial year, resulting in a £1.28m operating profit before interest and partners’ drawings.
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