Donors will be encouraged to buy a brick for the building by texting 'brick' to 88100. The £1.50 cost of the text will be used for the building fund.
The facility will provide accommodation for up to 120 families who live within walking distance of University College London Hospital. The visitors of patients will be offered 'something better than a fold-down bed or armchair'. A site has yet to be determined.
In-house PR focusing on London and the South-East will complement a billboard ad campaign devised by marcoms shop Catalyst.
PR manager Karen Baxter said: 'The outdoor media will go live on Monday [8 May], so media relations will also begin next week.
'The campaign will be tied in with the Biba Ball at London's Victoria & Albert Museum on 11 May, which is also raising funds for the appeal,' she added.
The charity has no definite timeline for the appeal, but the campaign could continue 'until at least the end of the summer', Baxter said.
The project was devised after CLIC Sargent was alerted to the plight of local families with nowhere to stay overnight when visiting children at UCLH.