I would normally scarify & apply weed & feed to my lawn at this time of year. But as we have had an extension built and have bare patches where trenches etc have been dug how should i go about repairing it this year?
Be very careful with weed killers, whether they are packaged separately or combined as in weed & feed products. They can slow or stop the growth of new grass.
1) Make sure the soil has fully settled in the trenches. It would be hard to fix if they continued to settle after the lawn started filling in.
2) If you need to fill the trenches a bit more use some soil conditioner or compost to blend with your soil. Bring them up level to the surrounding soil.
3) Get some grass seed of the same type as the rest of the lawn to scatter in the bare areas. Follow the directions about how much to use.
4) Cover the seed with a thin layer of fine compost or soil conditioner. Just enough to hide it from the birds, and hold the moisture next to the seed. If it is a large area I would then use a roller to make sure the seed and topping is in good contact with the soil. If it is a small area you might be able to tamp it with your foot enough to do the job.
5) Fertilize the whole lawn with a starter fertilizer (low nitrogen, moderate phosphate and potassium). This will help the seeds get going and not risk using a weed & feed.
6) Water the bare areas carefully so you do not wash away the seed or compost.
I would remove the moss. Give the grass seed the best chance of being in contact with good soil and the least competition for water and nutrients while it is young.
If moss is thriving in the area where you want lawn perhaps the area is too wet or too shaded for most lawn grasses. Most lawn grasses are best for full sun and some drying between watering.
Most mosses thrive in shade and constantly moist conditions.