You can extend a house in Lymington, but a good deal of the housing here comes with a planning officer effectively attached to it. The town carries 266 listed buildings and three conservation areas, along with one scheduled monument, which is a heavy heritage load for a place this size. None of that makes an extension impossible, and the town runs through around three hundred planning applications a year, so work plainly does get done. It just means more of the older and prettier stock is the kind you have to ask about before you touch, and asking is slower and dearer than not having to.
Start with listing, because that is the tightest control. Of the 266 listed buildings, none are the top Grade One tier, about fourteen are the middle Grade Two Star, and the remaining bulk, more than two hundred and fifty of them, are Grade Two. Grade Two is the most common and the least restrictive of the listings, but least restrictive is not the same as unrestricted. A listed building needs listed building consent for changes that affect its character, inside as well as out, and that reaches things an owner would never file under structural: a fireplace, a staircase, a run of original windows. Buy one of these and your freedom to alter it is not yours alone to decide.
Then there are the three conservation areas, taking in Lymington itself, Buckland and King's Saltern. A conservation area does not freeze a house the way listing can, but it tightens the rules around it. Some of the changes you could make freely elsewhere need consent inside the boundary, demolition is controlled, and work to trees comes with its own notice requirements. The effect is that even an unlisted house on a conservation-area street carries strings the same house two roads outside the line would not.
In practice this splits the town into two experiences. On an ordinary, unlisted house well outside the conservation boundaries, the usual permitted development freedoms apply and a modest extension is the routine job it is anywhere else. On a listed house, or one inside a conservation area, the same job starts with consent, drawings, and a wait, and the answer can come back as a no or a heavily conditioned yes. The most valuable thing you can do before buying is check the exact status of the exact property, because the gap between the two is the gap between a weekend project and a year-long negotiation.
On the freedom to extend or alter a home, Lymington scores a below-average two out of five, and the score is earned. The heritage that makes the town worth living in is the same heritage that puts a gate in front of your building plans, so much of the nicest stock here is bought on the understanding that the house is partly the town's to look after, not solely yours to change.