Part exchange valuation less than anticipated

Hi All,

We've had our house on the market for almost two weeks now, had 5 viewing in the first week but it been quiet this week. Most viewers said that really liked the property and complimented the size and decor but unfortunately weren't in a position to make an offer just yet.

The house is on the market for £335K, 3 bed detached in the north west.
I was under the impression that it was likely priced right due to having the immediate interest.

For context, we have extended the kitchen into the garage and built a summerhouse decked part of the garden and left plenty of lawn and created a raised border.
A similar property sold for £315K last month but it has had no work done to it at all and is also situated where parking is a problem and sits on a junction that sees almost all traffic in/out the estate. Another property facing us sold for £290K last year but again had no work done and they wanted a quick sale as they had to move further north for family.

We have been considering a part exchange were the new developer told us that they could only give a maximum of £318.5K, which we thought we'd accept, as its less hassle and we could maybe squeeze some incentives to bridge the gap.

They have come back today with a part exchange offer of £295K plus a cash incentive of 17.5K.
Combined this is 6K less than we where anticipating and with no incentives such as flooring, turf etc.

It's less than what the other property has just sold for but I would have expected with the extension of the kitchen and garden renovations it'd have added value, maybe i'm being naive?

What would you do in this situation?
I'm quite impulsive and would take the hit, but i need to convince the wife!