The group is acquiring LSH in a pre-pack administration deal that wipes out debts left over from an ill-fated management buy-out just before the market crashed.
But current shareholders will lose out as the majority of the purchase price will be used to pay off a private equity group which owns the debt.
Countrywide's decision is a vote of confidence in the commercial property sector, which has been badly hit by the downturn.
The group, which currently focuses on residential property, has more than 1,300 branches throughout the UK with brands including Bairstow Eves, Gascoigne-Pees and Hamptons International.
Management sees the purchase of LSH, which recently handled the sale of BBC Television Centre for £200 million, as an opportunity to cash in on expected growth in the commercial market.
Executives believe that while it has struggled with debts dating from the time of the £46 million buy-out in 2007, LSH's operational performance remains robust.
Countrywide said: "There is clear growth potential in the UK commercial property market. The company also believes that activity in the commercial markets is starting to improve and now is the right time to build on its position in his market."
LSH has a range of leading clients across the banking, finance, transport, telecoms and retail sectors, including more than half of all London borough councils. It recently handled the sale of 800 former railway sites raising £300 million.
Accounts for last year showed an operating profit of £5 million on sales of £64.1 million. Gross assets as of last month were £17.7 million.
Founded in 1773, it is described as one of the largest commercial property consultancies in the UK and Ireland with 26 offices and 861 employees.
Countrywide is acquiring LSH after reaching an agreement with Sankaty, which owned the company's debt, and involves LSH first going into administration.
A source close to the deal admitted shareholders would be hit as the bulk of the £34.1 million purchase price would go to Sankaty.
Countrywide said that as part of the deal, LSH would be discharged from any legacy debt from the management buy-out and placed "on a firm financial footing". It is subject to approval by the Financial Conduct Authority.
The move is the latest sign of the growing appetite among investors for a slice of the property market, buoyed up by low interest rates and Government initiatives.
Countrywide returned to the London stock market earlier this year in a flotation which valued the group at more than £800 million.
Last week, rival Foxtons saw its value increase by more than £100 million in just a few hours when it launched a share sale.