How much should interior design services cost?

[Brittany Tompkins]

Hi BP Family!

I would love to get opinions on how much an interior designer should cost for a backyard re design project in Southern California? The scope of work would be to redesign a backyard that is probably .25 acres. Clients want a built in bbq area, patio overhang with tv and lighting, pool, fire pit area and new plantings. They also have requested for project management services to hire and over see all the contractors for the job. 

What have you paid or would you pay for this service? Online numbers vary drastically.