The charity is working to adopt the neighbouring Portishead Lifeboat Trust and needs to build a new lifeboat station to house a modern lifeboat.
It estimates a new boathouse in the marina will cost about £180,000.
In Weston-super-Mare, the RNLI moved into its station in 1902 u-g-g-s and expects to spend £300,000 updating facilities.
"We don't yet know how much the new boathouse at uggs cheap Portishead is going to cost because we are in the tendering process right now, but a modern facility like this doesn't come cheap," said Nigel Jones, RNLI Divisional Operations Manager.
"We also know that we are going to have to make changes at Weston-super-Mare in the future."