The mailing included a compliments slip with text that appeared to be handwritten stating: "Re: Your home. Please call me as a matter of urgency quoting reference number XXX. Regards, Lesley Miller".
Two phone numbers were also included as well as small print which stated: "Haart is a trading name of Spicerhaart Limited...".
The mail-out, which appeared to be a hand-written letter, intended to encourage home-owners who already had their property on the market to sign up with Haart Estate Agents.
The advertiser said even though it received only a very small number of complaints, it would be willing to make changes to the envelope.
It said the mailing was only sent to consumers who were actively engaged in the house sale process in areas where Haart had an active branch presence.
It had not intended to mislead consumers and had believed it was evident the mailing was marketing material used to seek estate agency instructions.
The ASA noted the compliments slip inside the envelope displayed the Haart logo and branding as well as a reference to the instruction of agents in the small print.
However because the envelope - which included a postage stamp - and the letter appeared handwritten without any branding it could mislead consumers.
The watchdog welcomed Haart’s assurance that it would make changes in order to clarify that the mailing was marketing communication relating to a commercial interest in the sale of the recipients home.
The mailing must not be used again in its current form.