Should all garden lights operate from one control?
That depends on how the areas are used. A practical route and a decorative feature may benefit from separate operation. Explain the routines you want to support; the control arrangement and electrical requirements can then be considered by the appropriate installer.
Is adding a light the same as replacing a fitting?
A replacement may be able to use parts of an existing arrangement, while an additional light can introduce new cabling, mounting or controls. Both still need product and electrical suitability checked. Describe the intended function and show the existing installation without attempting to assess it yourself. Ask the quotation to distinguish fitting supply from investigation, installation, making good and the relevant testing or documentation.
Should decorative fittings be chosen before control requirements?
Consider both together. The desired effect and operation need to align with suitable products and installation.
Are motion sensors always the best choice for a garden?
They are useful for some access tasks, but position and behaviour matter. Unwanted activation or an unsuitable time setting can disturb a sitting area or neighbouring windows. Explain what should trigger the lights and how long you normally need them. Ask whether manual control or a different arrangement would suit that use better, then have the selected system specified and commissioned by the responsible installer.