Raise lintel in fireplace for cooker

You won't need planning permission. Application for building regulation approval may be be required. Contact your local council and speak to the building reg' department. They should be able to give you an answer over the phone. The job sounds straight forward. Masonry above the height of the new lintel will need to be supported. Ventilation depends on the type of appliance. You may need a flue liner with a cowl fitted to the chimney pot. If no gases are being produced then any flue is better ventilated with pepper pot cowl on the chimney with air bricks in the chimney breast. In summary, speak to the council. Then contact a builder, plumber (gas safe certified) or electrician - use My Builder - to do the work.